Daylight Lamps.
—The color of the light from an incandescent electric lamp depends on the temperature of the filament. In the case of the gas-filled Mazda lamp the high filament temperature produces a light that differs markedly from the vacuum lamps in that it contains a greater amount of blue and green rays. It is therefore possible to produce light that is the same as average daylight. Gas-filled lamps with globes colored to produce light of noonday quality are produced at an expenditure of 1.2 watts per candlepower.
In the matching of colors, it should be kept in mind that the tint of any color is influenced by the kind of light by which it is viewed. Colors matched by ordinary incandescent light containing a large percentage of red rays cannot produce the same effect when the same articles are seen in light of different quality. The daylight lamps are therefore intended to be used under conditions that require daylight quality.
Miniature Tungsten Lamps.
—The wonderful light-giving properties of tungsten has made possible the use of miniature incandescent lamps for an almost infinite variety of usages. The miniature lamps are similar in action to other incandescent electric lamps except that they are operated on voltages lower than is used on commercial circuits. When used on commercial circuits, incandescent tungsten lamps of less than 10 watts capacity require filaments that are too delicate to withstand the conditions of ordinary use. The properties of tungsten are such that the passage of only a small amount of current is required to render the filament incandescent. In the case of a 110-volt circuit, a 10-watt lamp requires only 0.09+ ampere to produce the desired incandescence. It will be remembered that the watt is a volt-ampere and the 10-watt lamp will then require
110 volts × 0.09 + ampere = 10 watts.
Since 10-watt lamps are the smallest units that may be used on 110-volt circuits, their employment in smaller sizes must be such as will give more stable filaments. This is possible when the lamps are used at lower voltage. A 10-watt lamp on a 10-volt circuit will require an ampere of current.
10 volts × 1 ampere = 10 watts.
A filament suitable for an ampere of current is shorter and heavier than that of the 110-volt lamp and therefore furnishes a good form of construction. Still lower voltages may be used with filaments suited to the quantity of light desired.
In the case of battery lamps that are intended to operate on 1 or more volts, the filaments are made in size and length to suit the condition of action. In all cases the product of the volts and amperes give the capacity of the lamp in watts.
Miniature lamps are ordinarily marked to show the voltage on which they are intended to operate. A 6-volt battery lamp is intended to be used with a primary battery of four to six cells depending on the condition of usage, or three cells of storage battery, each cell of which gives 2 volts of pressure.
Flash Lights.
—These are portable electric lamps composed of a miniature incandescent bulb, which with one or more dry cells are enclosed in a frame to suit the purpose of their use. They are made in pocket sizes or in form to be conveniently carried in the hand and are convenient and efficient lamps wherever a small amount of light is required for a short time. The electricity for operating the lamp is supplied by a battery of dry cells (to be described later), or by a single dry cell. In each case the incandescent bulb is suited to the voltage of the battery.
In replacing the bulbs care must be taken to see that the voltage is that suited to the battery. The voltage is usually stamped on the lamp base or marked on the bulb. In case a lamp intended for a single cell is used with a battery of three or four cells, the lamp filament will soon be destroyed. The reverse will be true should a lamp intended for a battery be used with a single cell. The single cell giving not much more than a volt of electromotive force will not send sufficient current through the lamp filament to render it incandescent.
The Electric Flat-iron.
—The changes that have been made in domestic appliances by the extended use of electricity have brought many innovations but none are more pronounced than the improvements made in the domestic flat-iron. It was the first of the household heating devices to receive universal recognition and its place as a domestic utility is firmly established.
The relatively high cost of heat as generated through electric energy is in a great measure counterbalanced in the flat-iron by high efficiency in its use. In the electric iron, the heat is developed in the place where it can be used to the greatest advantage, and transmitted to the face of the iron with but very little loss. Because of this direct application the cost of operation is but slightly in excess of the other methods of heating.
The electric flat-iron has now become a part of the equipment of every commercial laundry, where electricity can be obtained at a reasonable rate. The popularity of the electric iron is due to its cleanliness and to the increased amount of work that may be accomplished through its use. Because of the time saved in changing irons and the comfort of the room by reason of its lower temperature, a sufficiently greater amount of work is accomplished to more than compensate for the greater cost of heat.
The electric current is conducted to the flat-iron from the house circuit by wires made into the form of a flexible cord. The cord attaches to the electric-lamp fixture by a screw-plug and connects with the iron by a special attachment piece as indicated at P and R in Fig. 226. Connection is made to an incandescent lamp socket at any convenient place. The only precaution necessary in attaching the iron is to see that the fuse and the wires, which form the circuit, are of size sufficient to transmit the amount of current the iron is rated to use. As explained later, the fuse which is a part of every electric house circuit, and the conducting wires which form the heater circuit, must be sufficient in size to transmit the necessary current without material heating.
The cord connects with the socket at P, and the current turned on. It is attached with the iron by a piece R, made of non-conducting and heat-resisting material and arranged to make contact with the heater terminals by two brass plugs that are insulated from the body of the iron and afford easy means of making electric contact. The contact plugs are shown in Fig. 227. To make electric connection, the contact piece is simply pushed over the plugs, where it is held in place by friction. Instructions which accompany a flat-iron when purchased advise that the attachment piece be used in turning off the current. The reason for this is because of the flash that accompanies the break in the circuit when disconnection is made in the socket. This flash is really a small electric arc, that forms as the circuit is broken and which burns away the switch at the point of disconnection. The arc so formed burns away the contact pieces in the switch and it is soon destroyed. The attachment piece will stand this wear more readily than the socket switch and hence is preferable for disconnecting. The irons are frequently provided with a special switch for the service required in the flat-iron.
A spiral spring connected to the attachment cord prevents it from kinking when in use and thus breaking the conducting wires. The attachment cord is made of stranded wires to make it flexible. The strands of fine copper wire are made to correspond to the gage numbers by which the various sizes of wire are designated. In use the constant movement of the iron tends to kink the cord and thus breaks the strands. This action is most pronounced at the point where the cord attaches to the iron. For this reason a spiral spring wire encloses the cord for a short distance above the attachment piece. After long usage the cord is apt to break in this vicinity. It may usually be repaired by cutting off the ends of the cords and new connections made in the attachment piece. When the iron is in use the slack portion of the cord is kept from interfering with the work by the coiled wire S, which connects with the cord at any convenient place.
Electric flat-irons are made in a variety of styles and forms, the mechanism of each possessing some particular advantage, but all are provided with the same essential parts, chief of which is the heater with its electric attachment piece. In Fig. 228 is shown very clearly the construction of an example in which attention is called to the points of excellence that are required in a particularly serviceable iron. The form of the heating element which is recognized in the iron is also shown in Fig. 228.
In the figure the heater is made of coils of resistance wire, wound on a suitable frame of mica. The heating element is insulated from the body of the iron with sheets of mica, this being a material that makes an excellent insulator and is not materially affected by the heat to which it is subjected. The resistance wire of which the element is composed is especially prepared to resist the corroding action common to metal when heated in air. The form of the element is such as to permit the least movement of the turns of wire—in their constant heating and cooling—that will allow the different spires to make contact and thus change the resistance. Should the spires of wire come together, the current would be shunted across the contact and the resistance of the element decreased. The effect of such a reduction of resistance would be an increased flow of current and a corresponding increase of heat. In this, as in the electric lamp and all other electric circuits, the current, voltage and resistance follow the conditions of Ohm’s law.
Different sizes of irons will, of course, require different amounts of current. A 6-pound iron, such as is commonly used for household work, will take about 5 amperes of current at 110 volts pressure. The amount of electricity the iron is intended to consume is generally stamped on the nameplate of the manufacturer. This is specified by the number of volts and amperes of current the iron is rated to use. As an example, the iron may be marked, Volts 105-115, Amperes 2-3. This indicates that the iron is intended to be used on circuits that carry electric pressure varying from 105 to 115 volts and that the heater will use from 2 to 3 amperes of current, depending on the voltage.
To estimate the cost of operating such an iron, it is necessary to determine the number of watts of electric energy consumed. The number of watts of energy developed under any condition will be the product of the volts times the amperes. Suppose that in the above example the iron was used on a circuit of 110 volts. Under this condition the current required to keep the iron hot would be 2.5 amperes. The product of these two qualities, 110 × 2.5 is 275 watts. If the cost of electricity is 10 cents per kilowatt-hour (1000 watts) the cost of operating the iron would be
275⁄1000 × 10 cents = 2¾ cents an hour.
Since the electric iron requires a much larger amount of current than is usually required for ordinary lighting, the circuit on which it is used should receive more than passing attention. The wires should be of size amply large to carry without heating the current necessary for its operation. This topic will be discussed later but it is well here to call attention to the necessity for a circuit suited to the required current. If an iron requiring 5 amperes of current is attached to a circuit that is intended to carry only 3 amperes the conducting wires will be overheated and may be the cause of serious results.
The Electric Toaster.
—As shown in Fig. 229 the toaster is made of a series of heating elements mounted on mica frames and supported on a porcelain base. It is an example of heating by exposed wires and direct radiation. The heaters H are coils of flat resistance wire that are wound on wedge-shaped pieces of mica. They are supported on a wire frame that is formed to receive slices of bread on each side of the heaters. The attachment piece A and the material of the heater is similar in construction to that of the flat-iron. The electric circuit may be traced from the contacts at A and B in the attachment plug by the dotted lines which indicate the wires in the porcelain base. The current traverses each coil in turn and connects with the next, alternately at the top and bottom. The resistance is such as will permit the voltage of the circuit to send through the coils current sufficient to raise the heaters to a red heat. The added resistance of the hot wires decreases the flow of current to keep the temperature at the desired degree.
In a heater of this kind the resistance of the wire may increase with age and the coils fail to glow with a sufficient brightness. The reason for the lack of heat is that of decrease in current, due to the increased resistance of the wires. This condition may be corrected by the removal of a little of the heater coils. If a turn or two of the heater wire is removed, the resistance of the circuit is reduced and the effect of the increased current will produce a higher temperature in the heater.
Motors.
—As a means of developing mechanical power in small units, the electric motor has made possible its application in many household uses that were formerly performed entirely by manual labor. As a domestic utility electrical power is generated at a cost that is the least expensive of all its applications. As a means of lighting and heating electricity has had to compete with established methods and has won place because of the advantages it possesses over that of cost. In the development of domestic power it has practically no opponent. There is no other form of power that can be so successfully utilized in delivering mechanical work for the purposes required. A kilowatt of electric energy, for which 10 cents is a common price, will furnish a surprising amount of manual labor. Theoretically, 746 watts is equal to 1 horsepower. The commercial kilowatt is rated at an hour of time, and is, therefore, equal theoretically to 11⁄3 horsepower for one hour. While motors cannot be expected to transform all of this energy into actual work without loss, even at the low rate of efficiency attained by the small electric motor, they furnish power at a relatively small cost.
The first applications of electric power were those for sewing machines, fans, washing machines, etc. Its use has made possible the vacuum cleaner, automatic pumping, refrigeration, ventilation, and many other minor uses as the turning of ice-cream freezers, churning and rocking the cradle.
Electric motors are made in many sizes for power generation and in forms to suit any application. They are made to develop 1⁄30 horsepower and in other fractional sizes for both direct and alternating current.
In applying mechanical power to any particular purpose special appliances must be made to adopt electric motors to the required work. This is accomplished in all household requirements. The motors are made to run at a high rate of speed and must be reduced in motion by pulleys or gears to suit their condition of operation. As in the case of electric lamps they must be suited to the voltage and type of current of the circuit on which they are to be used.
Commercial electric circuits furnish electricity in two types, direct current, ordinarily termed D.C., and A.C. or alternating current. The terms direct and alternating current apply to the direction of the electric impulses which constitute the transmitted energy. In the electric dynamo, the generation of the current is due to impulses that are induced in the wires of the dynamo armature as they pass through a magnetic field of great intensity. These electric impulses are directed by the manner in which the wires cut across the lines of force which make up the magnetic field. In the case of the direct current the impulses are always in the same direction through the circuit, while in the other they are induced alternately to and fro and so produce alternating current.
The term electric current is used only for convenience of expressing a directed form of energy. Since nothing really passes through the wires but a wave of energy, the effect is the same whether the electric impulses are in the same or in opposite directions. An incandescent lamp will work equally well on an A.C. or a D.C. circuit of the proper voltage; but in the case of motors the form of construction must be suited to the kind of current. Both A.C. and D.C. commercial circuits are in common use, the units of measurement are the same for each but in ordering a motor it is necessary to state the type of current and the voltage, in order that the dealer may supply the required machine. In the case of an alternating motor it is further necessary to state the number of cycles of changes of direction made per second in the A.C. circuit. All of this information may be obtained by inquiring of a local electrician or of the power station from which the current is obtained.
There is still another item of information necessary to be supplied with an order for a motor, other than those of fractional horsepower. With motors of a horsepower or more it is necessary to state the number of phases included in the circuit. This information to be complete must state whether the motor is to operate on a single-phase, two-phase, or three-phase circuit. These terms apply to a condition made possible in A.C. generation that permits one, two, or three complete impulses to be developed in a circuit at the same time. These phases are transmitted by three wires, any two of which will form a circuit and give a supply of energy at the same voltage. Either one phase or all may be used at the same time and for this reason the phase of an A.C. motor should be given in an order. To make the information complete there should be included the number of cycles or complete electric impulses per second produced in the circuit. Suppose that a 1-horsepower motor is required to work on an A.C. circuit of 110 volts. Inquiry of the electric company reveals that the circuit is three-phase at 60 cycles per second. The dealer on receiving this information will be able to send a motor to suit your conditions. Most A.C. motors of 1 horsepower or less are of the single-phase variety. In the case of D.C. motors it is necessary only to state the voltage of the circuit to make the required information complete.
Fuse Plugs.
—Every electric circuit is liable to occurrences known as short-circuiting or “shorting.” This is a technical term describing a condition where, by accident or design, the wires of a circuit are in any way connected by a low-resistance conductor or by coming directly into contact with each other. In case of shorting, the resistance is practically all removed and the amount of current which flows through the circuit is so great as to produce a dangerous amount of heat in the wires. If the covering of a lamp cord becomes worn so as to permit the bare wire of the two strands to come together, a “short” is produced. Immediately, the reduced resistance permits the electric pressure to send an amount of current through the wires, greater than they are intended to carry. When this occurs an electric arc will form at the point of contact with the accompanying flash of vaporizing metal and the wire will finally burn off. Fires started from this cause are not uncommon.
To guard against accidents from short-circuiting, every electric circuit should be provided with fuses which, in cases of emergency, are intended to melt and thus break the circuit. Fuses are made of lead-composition or aluminum and are used in the form of wire or ribbon-like strips, of sizes that will carry a definite amount of current. They are designated by their carrying capacity in amperes. As an example: a 2-ampere fuse will carry 2 amperes of current without noticeable heating, but at a dangerous overload the fuse will melt and the circuit be broken. Should a short-circuit be formed at any time, the rush of current through the fuse will cause it almost immediately to melt, and stop the flow of current. They are, therefore, the safeguard of the circuit against undue heating of the conducting wires.
When an open fuse blows (melts), the heat generated by the arc, formed at the breaking circuit, is so sudden that there is frequently an explosive effect that throws the melted metal in all directions, and in case it comes into contact with combustible material a fire may result. To do away with this danger, fire insurance companies in their specifications of electric fixtures state what forms of fuses will be acceptable in the buildings to be insured. These specifications are known as the Underwriters Rules and may be obtained from any fire insurance company. The fuses, or fuse plug, as they are commonly called, generally occupy a place in a cabinet or distributing panel, near the point where the lead wires enter the building. The cabinet contains the porcelain cutouts for sending the current through the different circuits; the fuse plugs form a part of the cutouts, one fuse to each wire. The cabinet contains besides the cutouts a double-poled switch to be used for shutting off the current from the building when desired.
Cabinets for this purpose are made in standard form of wood or steel to suit the condition of service. These cabinets may be obtained from any dealers in electrical supplies or the cabinet may be made a part of the house since they are only small shallow closets. Fig. 230 represents such a cabinet as is used in the average dwelling. It is made of a light wooden frame set between the studding of a partition at any convenient place. The bottom of the cabinet is made sloping to prevent its being used as a place of storage for articles that might lead to trouble. The cabinet is sometimes lined with asbestos paper as a prevention from fire but this is not necessary as the fuse plugs and their receptacles, when of approved design, are sufficient to prevent accident.
The main wires which supply the house with electricity—marked lead wires—are brought into the cabinet as shown in Fig. 231 and attached to the poles of the switch S. In passing through the switch the lead wires each contain a mica-covered fuse plug F, that will be described later. The current at any time may be entirely cut off from the house by pulling the handle H, which is connected by an insulating bar and the contacts N of the switch. When the handle H is pulled to separate the contact pieces, all electric connection is severed at that point.
The wattmeter for measuring the current is placed at the points marked meter, as a part of the main circuit. The main wires in the cabinet terminate in the porcelain cutouts, from which are taken off the various circuits of the house. In the figure, three such cutouts are shown making three circuits marked 1, 2, and 3. In circuit No. 1, the fuses are marked F. These wires are joined to the main wires at the points marked C and C´. The number of circuits the house will contain depends on the number of lights and the manner in which they are placed. The circuits are intended to be arranged so that in case of a short, no part of the house will be left entirely in darkness.
Fuses for general use are made in two different types—the plug type and the cartridge type—each of which conforms to the rules of the Underwriters Association. Those most commonly used for house wiring are the plug type shown in Fig. 232 and indicated in the figure just described. These plugs are made of porcelain and provided with a screw base which permits their being screwed into place like an incandescent lamp. The front of the plug is arranged with a mica window which allows inspection to be made in case of a short, the blown fuse indicating the circuit in which the trouble is located. Another style of the same type of plug, known as the re-fusable fuse plug, permits the fuse to be replaced after the wire has been destroyed by a short.
The second type is commonly known as the cartridge fuse plug from its general appearance. This fuse is shown in Fig. 233. The fusable wire is enclosed in a composition fiber tube, the ends of which are covered by brass caps which afford contact pieces in the fuse receptacle and to which are fastened the ends of the fuse wire. These fuses are very generally employed in power circuits and others of large current capacity. The small circle in the center of the label is the indicator. When the fuse burns out, a black spot will appear in the circle. It is sometimes desirable to use the cartridge fuse plug in receptacles intended for the mica-covered type. The use of the cartridge fuses under this condition is effected by use of a porcelain receptacle such as is shown in Fig. 234; the cartridge fuse is simply inserted into the receptacle which is then screwed into the socket in place of the mica fuse.
In order to avoid any possible chance of overloading the wires of a circuit, fuses are installed which are suited to the work to be performed. Suppose that there are ten 40-watt lamps that may be used on a circuit, each lamp of which requires 4⁄11 ampere of current.
110 × C = 40 watts
C = 40⁄110 = 4⁄11 ampere per lamp.
Ten such lamps require ten times 4⁄11 ampere or 40⁄11 = 3.7 amperes to supply the lamps.
A fuse that will carry 3.7 amperes of current will supply the circuit but a 5-ampere fuse will permit an increase in the size of the lamp and will fulfill all the necessary conditions. If, however, an electric heater requiring 7 amperes were attached to the circuit, the fuse being intended for only 5 amperes would soon burn out. When a fuse burns out it must be replaced either with an entirely new receptacle or the fuse wire must be replaced.
It sometimes happens that in case of a blown fuse there is no extra part at hand and a wire of much greater carrying capacity is used in its place. It should be remembered that in this practice of “coppering” a blown fuse, has taken away the protection against short-circuiting with its possibility of mischief.
When a short occurs, the cause should be sought for. It cannot be located and on being replaced a second fuse blows, the services of an electrician should be secured.
Electric Heaters.
—All electric heating devices—whether in the form of hot plates, ovens, stoves or other domestic heating apparatus-possess heating elements somewhat similar to the flat-iron or the toaster. The construction of the heating element will depend on the use for which the heater is intended and the temperature to be maintained. Hot plates similar to that of Fig. 235 are made singly or two or more in combination. When the heat is to be transmitted directly by radiation the heating coils are open, as with the toaster. Under other conditions the coils are embedded in enamel that is fused to a metal plate. In elements of this kind the heat is transmitted to the plate entirely by conduction from which it is utilized in any manner requiring a heated surface. The form of the heating element will, therefore, depend on the application of the heat, whether it is by direct radiation or by a combination of radiation and conduction.
Electric ovens are constructed to utilize electric heat in an insulated enclosure. Heat derived from electricity is more expensive than from other sources but when used in insulated ovens it may be made to conveniently perform the service of that derived from other fuels. In electric ovens the heaters are attached to inside walls. As in other heating elements they are arranged to suit the conditions for which the oven is to be used. The heaters are usually so divided as to permit either all of the heaters to be used at the same time to quickly produce a high temperature, or only a portion of the heat to be used in keeping up the temperature lost by radiation. Ovens of this kind may be provided with regulators by means of which the heat may be automatically kept at any desired temperature. Such heating and temperature regulation may be used to produce any desired condition, but in practice the cost of the heat is the factor which determines its use. Unless electric heat is conserved by insulation it cannot become a competitor with other forms of heating.
Electric cooking stoves and ranges are made for every form of domestic and culinary service. They fulfill many purposes that may be obtained in no other way. As conveniences, the cost of heat becomes of secondary consideration and their use is constantly increasing. In Fig. 236 is an example of a time-controlled and automatically regulated electric range. In the picture is shown separately all of the heaters for the ovens and stove top. The part S shows the switches attached to the heaters of the stove top, which is raised to show the connecting wires. In the larger oven there are two heaters of 1000 watts each, and in the smaller oven one heater of 850 watts. Each heater may be controlled separately with a switch giving three regulations of heat—high, medium and low. The advantage of this arrangement lies in the fact that one can set the two heaters in the oven at different temperatures which will permit either a slow or quick heat, but when the predetermined temperature is reached the current will be automatically cut off by the circuit-breakers. Such flexibility of heat control in the ovens permits the operator to apply heat at both top and bottom for baking and roasting at just the desired temperature. This arrangement also avoids the danger of scorching food from concentration of heat, and warping utensils or the linings of the oven. All oven heaters on the automatic ranges are further controlled and mastered by the circuit-breakers.
Intercommunicating Telephones.
—This form of telephone is used over short distances such as from room to room in buildings or for connecting the house with the stable, garage, etc. It is complete, in that it possesses the same features as any other telephone but the signal is an electric call-bell instead of the polarized electric bell used in commercial telephone service.
Any telephone is made to perform two functions: (1) that of a signal with which to call attention; and (2) the apparatus required to transmit spoken words. In the intercommunicating telephone or interphone, the signal is made like any call-bell and parts are similar to those described under electric signals. The bell-ringing mechanism is included in the box with the transmitting apparatus and the signal is made by pressing a push button. It is not suitable for connecting with public telephones. Telephone companies, as a rule, do not permit connection with their lines any apparatus which they do not control.
The interphone of Fig. 237 shows the instrument complete except the battery. This form of instrument is inexpensive, easy to put in, simple to operate and supplies a most excellent means of intercommunication. Complete directions for installation are supplied with the phones by the manufacturers.
Electric Signals.
—Electrical signaling devices for household use, in the form of bells and buzzers, are made in a great variety of forms and sizes to suit every condition of requirement. The vibrating mechanism of the doorbell is used in all other household signals except that of the magneto telephone. It is an application of the electromagnet, in which the magnetism is applied to vibrate a tapper against the rim of a bell.
A bell system consists of the gong with its mechanism for vibrating the armature, an electric battery or A.C. transformer connected to the magnet coils to form an electric circuit and a push button which serves to close the circuit whenever the bell is to be sounded. The bell system is an open-circuit form of apparatus; that is, the circuit is not complete except during the time the bell is ringing. By pressing the push button the circuit is closed and the electric current from the battery flows through the magnet and causes the tapper to vibrate. When the push button is released the circuit is broken and the circuit stands open until the bell is to be again used. The parts of the bell mechanism are shown in Fig. 238 where with the battery, the push button and the connecting wires is shown a complete doorbell outfit. These parts may be placed in different parts of the building and connected by wires as shown in the Fig. 239. The bell is located at R, in the kitchen. The battery is placed in the closet at B, the connecting wires are indicated by the heavy lines; they are secured to any convenient part of the wall and extend into the basement and are fastened to the joists. The wires terminate in the push button P, where they pass through the frame of the front door. The wires are secured by staples to keep them in place. Each wire is fastened separately to avoid the danger of short-circuiting. If both wires are secured with a single staple there is a possibility of the insulation being cut and a short produced across the staple.
The battery B, in Fig. 238, is a single dry cell but more commonly it is composed of two dry cells joined in series. It is connected, as shown in the figure, to the binding posts P1 and P2 of the vibrating mechanism, the push button PB serving to make contact when the circuit is to be closed. When the button is pressed the circuit is complete from the + pole of the battery cell through the binding post P1, across the contact F, through the spring A, through the magnet coils M, across the binding post P2 and push button to the-pole of the cell. The vibration of the tapper is caused by the magnetized cores of the coils M. When the electric current flows through the coils of wire, the iron cores become temporary magnets. This magnetism attracts the iron armature attached to the spring A, and it is suddenly pulled forward with energy sufficient to cause the tapper to strike the gong. As the armature moves forward, the spring contact at F is broken and the current stops flowing through the magnet coils. When the current ceases to flow in the magnet coils, the cores are demagnetized and the armature is drawn back by the spring A to the original position. As soon as the contact is restored at F a new impulse is received only to be broken as before. In this manner the bell continues ringing so long as the push button makes contact. The screw at F is adjusted to suit the contact with the spring attached to the armature. The motion of the armature may be regulated to a considerable degree by this adjustment. When properly set the screw is locked in place by a nut and should require no further attention.
Electric bells vary in price according to design and workmanship. A bell outfit may be purchased complete for $1 but it is advisable to install a bell of better construction, as few pieces of household mechanism repay their cost in service so often as a well-made bell. The bell should be rigid, well-constructed, and the contact piece F should be adjustable. This part F, being the most important of the moving parts of the bell, is shown separately in Fig. 240. Only the ends of the magnet coils with their cores are shown in the figure. The contact is made at A, by the pressure of the spring against the end of the adjustable screw D. When the screw is properly adjusted it is locked securely in place by the nut G. The screw D is held with a screw-driver and the nut G forced into position to prevent any movement. If the screw is moved, so that contact is lost at A, the bell will not ring. In the better class of bells the point of the screw and its contact at A are made of platinum to insure long life. With each movement of the armature a spark forms at the contact which wears away the point, so that to insure good service these points must be made of refractory material.
Buzzers.
—Electric bells are often objectionable as signal calls because of their clamor, but with the removal of the bell the vibrating armature serves equally well as a signal but without the undesirable noise. With the bell and tapper removed the operating mechanism of such a device works with a sound that has given to them the name of buzzers. Fig. 241 illustrates the form of an iron-cased buzzer for ordinary duty. The working parts are enclosed by a stamped steel cover that may be easily removed. The mechanism is quite similar to that already described in the doorbell and Fig. 240 shows in detail the working parts. The noise, from which the device takes its name, is produced by the armature and spring in making and breaking contact.
Burglar Alarms.
—A burglar alarm is any device that will give notice of the attempted entrance of an intruder. It is usually in the form of a bell or buzzer placed in circuit with a battery, as a doorbell system, in which the contact piece is placed to detect the opening of a door or window. The contact is arranged to start the alarm whenever the window or door is opened beyond a certain point. The attachment shown in Fig. 242 is intended to form the contact for a window. It is set in the window frame so that the lug C will be depressed and close the alarm circuit in case the sash is raised sufficiently to admit a man. Each window may be furnished with a similar device and the doors provided with suitable contacts which together form a system to operate in a single alarm. During the time when the alarm is not needed it is disconnected by a switch. The windows and doors are sometimes connected with an annunciator which will indicate the place from which an alarm is given. An annunciator used for this purpose designates the exact point at which the contact is made and removes the necessity of searching for the place of attempted entrance.
Fig. 241.—The electric buzzer.
Fig. 242.—Contact for a window burglar alarm.
Fig. 243.—Trip contact which announces the opening of a door.
Fig. 244.—Contact for a door alarm.
Fig. 245.—Doorway or hall matting with contacts for electric alarm.
In Fig. 243 is illustrated one form of door trip which may be used on a door to announce its opening. This trip makes electric connection in the alarm circuit when the opening door comes into contact with the swinging piece T, but no contact is made as the door closes. The trip is fastened with screws at D to the frame above the door. The opening door comes into contact with T and moves it forward until the electric circuit is formed at C; after the door has passed, a spring returns it to place. As the door is closed, the part T is moved aside without making electric contact.
Fig. 244 is another form of door alarm that makes contact when the door is opened and remains in contact until the door is closed. The part P is set into the door frame of the door in such position that the contact at C is held open when the door is closed. When the door is opened a spring in C closes the contact and causes the alarm to sound. It continues to sound until the door is closed and the contact is broken. When the use of the alarm is not required, the contact-maker is turned to one side and the contact is held open by a catch. It is put out of use by pressing the plunger to one side.
The matting shown in Fig. 245 is provided with spring contacts so placed that no part may be stepped upon without sounding the alarm. When placed in a doorway and properly connected with a signal, no person can enter without starting an alarm. The matting is attached to the alarm by the wires C and contacts are set at close intervals so that a footstep on the mat must close at least one contact.
Annunciators.
—It is often convenient for a bell or buzzer to serve two or more push buttons placed in different parts of the house. In order that there may be means of designating the push button used—when the bell is rung—an annunciator is provided. This is a box arranged with an electric bell and the required number of pointers and fingers corresponding to the push buttons. In Fig. 246 is shown an annunciator with which two push buttons are served by the single bell. The annunciator is placed at the most convenient place of observation, usually in the kitchen. When the bell rings the pointer indicates the push button that has last been used. In hotels or apartment houses an annunciator with a single bell may thus serve any number of push buttons. In a burglar-alarm system the annunciator numbers are arranged to indicate the windows and other openings at which entrance might be made. When the alarm sounds the annunciator indicates the place from which the alarm is made.