Things a Boy Should Know About Electricity / Second Edition

CHAPTER XXII.
HOW LIGHT IS PRODUCED BY THE ARC LAMP.

152. The Electric Arc. When a strong current passes from one carbon rod to another across an air-space, an electric arc is produced. When the ends of two carbon rods touch, a current can pass from one to the other, but the imperfect contact causes resistance enough to heat the ends red-hot. If the rods be separated slightly, the current will continue to flow, as the intensely heated air and flying particles of carbon reduce the resistance of the air-space.

As the electric arc is extremely hot, metals are easily vaporized in it; in fact, even the carbon rods themselves slowly melt and vaporize. This extreme heat is used for many industrial purposes.

"The phenomenon of the electric arc was first noticed by Humphrey Davy in 1800, and its explanation appears to be the following: Before contact the difference of potential between the points is insufficient to permit a spark to leap across even 1/10000 of an inch of air-space, but when the carbons are made to touch, a current is established. On separating the carbons, the momentary extra current due to self-induction of the circuit, which possesses a high electromotive force, can leap the short distance, and in doing so volatilizes a small quantity of carbon between the points. Carbon vapor, being a partial conductor, allows the current to continue to flow across the gap, provided it be not too wide; but as the carbon vapor has a very high resistance it becomes intensely heated by the passage of the current, and the carbon points also grow hot. Since, however, solid matter is a better radiator than gaseous matter, the carbon points emit far more light than the arc itself, though they are not so hot. It is observed, also, that particles of carbon are torn away from the + electrode, which becomes hollowed out to a cup-shape, and some of these are deposited on the - electrode."

153. Arc Lamps. As the carbons gradually wear away, some device is necessary to keep their ends the right distance apart. If they are too near, the arc is very small; and if too far apart, the current can not pass and the light goes out. The positive carbon gives the more intense light and wears away about twice as fast as the - carbon, so it is placed above the - carbon, to throw the light downwards.

Arc lamps contain some device by which the proper distance between the carbons can be kept. Most of them grip the upper carbon and pull it far enough above the lower one to establish the arc. As soon as the distance between them gets too great again, the grip on the upper carbon is loosened, allowing the carbon to drop until it comes in contact with the lower one, thus starting the current again. These motions are accomplished by electromagnets. Fig. 207 shows a form of arc lamp with single carbons that will burn from 7 to 9 hours.

Fig. 208 shows the mechanism by which the carbons are regulated. Fig. 209 shows a form of double carbon, or all-night lamp, one set of carbons being first used, the other set being automatically switched in at the proper time.

Figs. 210, 211 show forms of short arc lamps, for use under low ceilings, so common in basements, etc.

Fig. 212 shows a hand-feed focussing type of arc lamp. In regular street lamps, the upper carbon only is fed by mechanism, as it burns away about twice as fast as the lower one, thus bringing the arc lower and lower. When it is desired to keep the arc at the focus of a reflector, both carbons must be fed.

Fig. 213 shows a theatre arc lamp, used to throw a strong beam of light from the balcony to the stage.

Fig. 214 shows the arc lamp used as a search-light. The reflector throws a powerful beam of light that can be seen for miles; in fact, the light is used for signalling at night. Fig. 215 shows how search-lights are used at night on war-vessels.

CHAPTER XXIII.
X-RAYS, AND HOW THE BONES OF THE HUMAN BODY ARE PHOTOGRAPHED.

154. Disruptive Discharges. We have seen, in the study of induction coils, that a spark can jump several inches between the terminals of the secondary coil. The attraction between the two oppositely charged terminals gets so great that it overcomes the resistance of the air-space between them, a brilliant spark passes, and they are discharged. This sudden discharge is said to be disruptive, and it is accompanied by a flash of light and a loud report. The path of the discharge may be nearly straight, or crooked, depending upon the nature of the material in the gap between the terminals.

155. Effect of Air Pressure on Spark. The disruptive spark takes place in air at ordinary pressures. The nature of the spark is greatly changed when the pressure of the air decreases. Fig. 216 shows an air-tight glass tube so arranged that the air can be slowly removed with an air-pump. The upper rod shown can be raised or lowered to increase the distance between it and the lower rod, these acting as the terminals of an induction coil. Before exhausting any air, the spark will jump a small distance between the rods and act as in open air. As soon as a small amount of air is removed, a change takes place. The spark is not so intense and has no definite path, there being a general glow throughout the tube. As the air pressure becomes still less, the glow becomes brighter, until the entire tube is full of purple light that is able to pass the entire length of it; that is, the discharge takes place better in rarefied air than it does in ordinary air.

156. Vacuum-Tubes. As electricity passes through rarefied gases much easier than through ordinary air, regular tubes, called vacuum-tubes, are made for such study. Fig. 217 shows a plain tube of this kind, platinum terminals being fused in the glass for connections. These tubes are often made in complicated forms, Fig. 218, with colored glass, and are called Geissler tubes. They are often made in such a way that the electrodes are in the shape of discs, etc., and are called Crookes tubes, Fig. 219. A slight amount of gas is left in the tubes.

157. Cathode Rays. The cathode is the electrode of a vacuum-tube by which the current leaves the tube, and it has been known for some time that some kind of influence passes in straight lines from this point. Shadows, Fig. 219, are cast by such rays, a screen being placed in their path.

158. X-Rays. Professor Roentgen of Würzburg discovered that when the cathode rays are allowed to fall upon a solid body, the solid body gives out still other rays which differ somewhat from the original cathode rays. They can penetrate, more or less, through many bodies that are usually considered opaque. The hand, for example, may be used as a negative for producing a photograph of the bones, as the rays do not pass equally well through flesh and bone.

Fig. 220 shows a Crookes tube fitted with a metal plate, so that the cathode rays coming from C will strike it. The X-rays are given out from P. These rays are invisible and are even given out where the cathode rays strike the glass. Some chemical compounds are made luminous by these rays; so screens are made and coated with them in order that the shadows produced by the X-rays can be seen by the eye. Professor Roentgen named these the X-rays. Fig. 220-A shows a fluoroscope that contains a screen covered with proper chemicals.

159. X-Ray Photographs. Bone does not allow the X-rays to pass through it as readily as flesh, so if the hand be placed over a sensitized photographic plate, Fig. 221, and proper connections be made with the induction coil, etc., the hand acts as a photographic negative. Upon developing the plate, as in ordinary photography, a picture or shadow of the bones will be seen. Fig. 222 shows the arrangement of battery, induction coil, focus tube, etc., for examining the bones of the human body.

CHAPTER XXIV.
THE ELECTRIC MOTOR, AND HOW IT DOES WORK.

160. Currents and Motion. We have seen, Chapter XII., that when coils of wire are rapidly moved across a strong magnetic field, a current of electricity is generated. We have now to deal with the opposite of this; that is, we are to study how motion can be produced by allowing a current of electricity to pass through the armature of a machine.

Fig. 224 shows, by diagram, a coil H, suspended so that it can move easily, its ends being joined to a current reverser, and this, in turn, to a dry cell D C. A magnet, H M, will attract the core of H when no current passes. When the current is allowed to pass first in one direction and then in the opposite direction, by using the reverser, the core of H will jump back and forth from one pole of H M to the other. There are many ways by which motion can be produced by the current, but to have it practical, the motion must be a rotary one. (See "Study," Chapter XXVI., for numerous experiments.)

161. The Electric Motor is a machine for transforming electric energy into mechanical power. The construction of motors is very similar to that of dynamos. They have field-magnets, armature coils, commutator, etc.; in fact, the armature of an ordinary direct current dynamo will revolve if a current be passed through it, entering by one brush and leaving by the other. There are many little differences of construction, for mechanical and electrical reasons, but we may say that the general construction of dynamos and motors is the same.

Fig. 225 shows a coil of wire, the ends of which are connected to copper and zinc plates. These plates are floated in dilute sulphuric acid, and form a simple cell which sends a current through the wire, as shown by the arrows.

We have seen that a current-carrying wire has a magnetic field and acts like a magnet; so it will be easily seen that if a magnet be held near the wire it will be either attracted or repelled, the motion depending upon the poles that come near each other. As shown in the figure, the N pole of the magnet repels the field of the wire, causing it to revolve. We see that this action is just the reverse to that in galvanometers, where the coil is fixed, and the magnet, or magnetic needle, is allowed to move. As soon as the part of the wire, marked A in Fig. 225, gets a little distance from the pole, the opposite side of the wire, B, begins to be attracted by it, the attraction getting stronger and stronger, until it gets opposite the N pole. If the N pole were still held in place, B would vibrate back and forth a few times, and finally come to rest near the pole. If, however, as soon as B gets opposite N the S pole of the magnet be quickly turned toward B, the coil will be repelled and the rotary motion will continue.

Let us now see how this helps to explain electric motors. We may consider the wire of Fig. 225 as one coil of an armature, and the plates, C and Z, as the halves of a commutator. In this arrangement, it must be noted, the current always flows through the armature coil in the same direction, the rotation being kept up by reversing the poles of the field-magnet. In ordinary simple motors the current is reversed in the armature coils, the field-magnets remaining in one position without changing the poles. This produces the same effect as the above. The current is reversed automatically as the brushes allow the current to enter first one commutator bar and then the opposite one as the armature revolves. The regular armatures have many coils and many commutator bars, as will be seen by examining the illustrations shown.

The ordinary galvanometer may be considered a form of motor. By properly opening and closing the circuit, the rotary motion of the needle can be kept up as long as current is supplied. Even an electric bell or telegraph sounder may be considered a motor, giving motion straight forward and back.

162. The Uses of Motors are many. It would be impossible to mention all the things that are done with the power from motors. A few illustrations will give an idea of the way motors are attached to machines.

163. Starting Boxes. If too much current were suddenly allowed to pass into the armature of a motor, the coils would be over-heated, and perhaps destroyed, before it attained its full speed. A rapidly revolving armature will take more current, without being overheated, than one not in motion. A motor at full speed acts like a dynamo, and generates a current which tends to flow from the machine in a direction opposite to that which produces the motion. It is evident, then, that when the armature is at rest, all the current turned on passes through it without meeting with this opposing current.

Fig. 235 shows a starting, stopping, and regulating box, inside of which are a number of German-silver resistance coils properly connected to contact-points at the top. By turning the knob, the field of the motor is immediately charged first through resistance, then direct, and then the current is put on the armature gradually through a series of coils, the amount of current depending upon the distance the switch is turned. Fig. 236 shows a cross section of the same.

CHAPTER XXV.
ELECTRIC CARS, BOATS, AND AUTOMOBILES.

164. Electric Cars, as well as boats, automobiles, etc., etc., are moved by the power that comes from electric motors, these receiving current from the dynamos placed at some "central station." We have already seen how the motor can do many kinds of work. By properly gearing it to the car wheels, motion can be given to them which will move the car.

Fig. 237 shows two dynamos which will be supposed to be at a power house and which send out a current to propel cars. From the figure it will be seen that the wires over the cars, called trolley-wires, are connected to the positive (+) terminals of the dynamos, and that the negative (-) terminals are connected to the tracks. In case a wire were allowed to join the trolley-wire and track, we should have a short circuit, and current would not only rush back to the dynamo without doing useful work, but it would probably injure the machines. When some of the current is allowed to pass through a car, motion is produced in the motors, as has been explained. As the number of cars increases, more current passes back to the dynamos, which must do more work to furnish such current.

Trolley-poles, fastened to the top of the cars and which end in grooved wheels, called trolley-wheels, are pressed by springs against the trolley-wires. The current passes down these through switches to controllers at each end of the car, one set being used at a time.

165. The Controllers, as the name suggests, control the speed of the car by allowing more or less current to pass through the motors. The motors, resistance coils and controllers are so connected with each other that the amount of current used can be regulated.

When the motorman turns the handle of the controller to the first notch, the current passes through all of the resistance wires placed under the car, then through one motor after the other. The motors being joined in series by the proper connections at the controller, the greatest resistance is offered to the current and the car runs at the slowest speed at this first notch. As more resistance is cut out by turning the handle to other notches, the car increases its speed; but as the resistance wires become heated and the heat passes into the air, there is a loss of energy. It is not economical to run a car at such a speed that energy is wasted as heat. As soon as the resistance is all cut out, the current simply passes through the motors joined in series. This gives a fairly slow speed and one that is economical because all the current tends to produce motion.

By allowing the current to pass through the motors joined in parallel, that is, by allowing each to take a part of the current, the resistance is greatly reduced, and a higher speed attained. This is not instantly done, however, as too much strain would be put upon the motors. As soon as the next notch is reached, the motors are joined in parallel and the resistance also thrown in again. By turning the handle still more, resistance is gradually cut out, and the highest speed produced when the current passes only through the motors in parallel.

Fig. 238 represents a controller, by diagram, showing the relative positions of the controller cylinder, reversing and cut-out cylinders, arrangements for blowing out the short electric arcs formed, etc. A ratchet and pawl is provided, which indicates positively the running notches, at the same time permitting the cylinder to move with ease. Fig. 239 shows a top view of the controller.

166. Overhead and Underground Systems. When wires for furnishing current are placed over the tracks, as in Fig. 237, we have the overhead system. In cities the underground system is largely used. The location of the conducting wires beneath the surface of the street removes all danger to the public, and protects them from all interference, leaving the street free from poles and wires.

Fig. 240 shows a cross-section of an underground conduit. The rails, R R, are supported by cast-iron yokes, A, placed five feet apart, and thoroughly imbedded in concrete. The conduit has sewer connections every 100 feet. Conducting bars, C C, are placed on each side of the conduit, and these are divided into sections of about 500 feet. Insulators, D D, are placed every 15 feet. They are attached to, and directly under, the slot-rails, the stem passing through the conductor bar.

Figs. 240 and 241 show the plow E. The contact plates are carried on coiled springs to allow a free motion. Two guide-wheels, F F, are attached to the leg of the plow. The conducting wires are carried up through the leg of the plow.

167. Appliances. A large number of articles are needed in the construction of electric railroads. A few, only, can be shown that are used for the overhead system. Fig. 242 shows a pole insulator. Fig. 243 shows a feeder-wire insulator. Fig. 244 shows a line suspension. Fig. 245 shows a form of right-angle cross which allows the trolley-wheels of crossing lines to pass. Fig. 246 shows a switch. In winter a part of the current is allowed to pass through electric heaters placed under the seats of electric cars.

168. Electric Boats are run by the current from storage batteries which are usually placed under the seats. An electric motor large enough to run a small boat takes up very little room and is generally placed under the floor. This leaves the entire boat for the use of passengers. The motor is connected to the shaft that turns the screw. Fig. 247 shows one design.

169. Electric Automobiles represent the highest type of electrical and mechanical construction. The running-gear is usually made of the best cold-drawn seamless steel tubing, to get the greatest strength from a given weight of material. The wheels are made in a variety of styles, but nearly all have ball bearings and pneumatic tires. In the lightest styles the wheels have wire spokes.

The electric motors, supported by the running-gear, are geared to the rear wheels. The motors are made as nearly dust-proof as possible.

Storage batteries are put in a convenient place, depending upon the design of the carriage, and from these the motors receive the current. These can be charged from the ordinary 110-volt lighting circuits or from private dynamos. The proper plugs and attachments are usually furnished by the various makers for connecting the batteries with the street current, which is shut off when the batteries are full by an automatic switch.

Controllers are used, as on electric cars, the lever for starting, stopping, etc., being usually placed on the left-hand side of the seat. The steering is done by a lever that moves the front wheels. Strong brakes, and the ability to quickly reverse the motors, allow electric carriages to be stopped suddenly in case of accidents.

Electric automobiles are largely used in cities, or where the current can be easily had. The batteries must be re-charged after they have run the motors for a certain time which depends upon the speed and road, as well as upon the construction. Where carriages are to be run almost constantly, as is the case with those used for general passenger service in cities, duplicate batteries are necessary, so that one or two sets can be charged while another is in use. Fig. 248 shows one form of electric vehicle, the storage batteries being placed under and back of the seat.

CHAPTER XXVI.
A WORD ABOUT CENTRAL STATIONS.

170. Central Stations, as the word implies, are places where, for example, electricity is generated for the incandescent or arc lights used in a certain neighborhood; where telephone or telegraph messages are sent to be resent to some other station; where operators are kept to switch different lines together, so that those on one line can talk to those on another, etc., etc. There are many kinds of central stations, each requiring a large amount of special apparatus to carry on the work. Fig. 249 gives a hint in regard to the way car lines get their power from a central power station. As a large part of the apparatus required in ordinary central stations has already been described, it is not necessary to go into the details of such stations.

In lighting stations, for example, we have three principal kinds of apparatus. Boilers produce the steam that runs the steam engines, and these run the dynamos that give the current. Besides these there are many other things needed. The electrical energy that goes over the wires to furnish light, heat, and power, really comes indirectly from the coal that is used to boil water and convert it into steam. The various parts of the central station merely aid in this transformation of energy.

The dynamos are connected to the engines by belts, or they are direct connected. Figs. 250, 251, show dynamos connected to engines without belts.

The current from the dynamos is led to large switchboards which contain switches, voltmeters, ammeters, lightning arresters, and various other apparatus for the proper control and measurement of the current. From the switchboard it is allowed to pass through the various street mains, from which it is finally led to lamps, motors, etc.

Water-power is frequently used to drive the dynamos instead of steam engines. The water turns some form of water-wheel which is connected to the dynamos. At Niagara Falls, for example, immense quantities of current are generated for light, heat, power, and industrial purposes.

CHAPTER XXVII.
MISCELLANEOUS USES OF ELECTRICITY.

171. The Many Uses to which the electric current is put are almost numberless. New uses are being found for it every day. Some of the common applications are given below.

172. Automatic Electric Program Clocks, Fig. 252, are largely used in all sorts of establishments, schools, etc., for ringing bells at certain stated periods. The lower dial shown has many contact-points that can be inserted to correspond to given times. As this revolves, the circuits are closed, one after the other, and it may be so set that bells will be rung in different parts of the house every five minutes, if desired.

173. Call Boxes are used to send in calls of various kinds to central stations. Fig. 253 shows one form. The number of different calls provided includes messenger, carrier, coupé, express wagon, doctor, laborer, police, fire, together with three more, which may be made special to suit the convenience of the individual customer. The instruments are provided with apparatus for receiving a return signal, the object of which is to notify the subscriber that his call has been received and is having attention.

Fig. 254 shows another form of call box, the handle being moved around to the call desired. As it springs back to the original position, an interrupted current passes through the box to the central station, causing a bell to tap a certain number of times, giving the call and location of the box.

174. Electric Gas-Lighters. Fig. 255 shows a ratchet burner. The first pull of the chain turns on the gas through a four-way gas-cock, governed by a ratchet-wheel and pawl. The issuing gas is lighted by a wipe-spark at the tip of the burner. Alternate pulls shut off the gas. As the lever brings the attached wire A, in contact with the wire B, a bright spark passes, which ignites the gas, the burner being joined with a battery and induction or spark coil.

Automatic burners are used when it is desired to light gas at a distance from the push-button. Fig. 256 shows one form. Two electromagnets are shown, one being generally joined to a white push-button for turning on the gas and lighting it, the other being joined to a black button which turns off the gas when it is pressed. The armatures of the magnets work the gas-valve. Sparks ignite the gas, as explained above.

175. Door Openers. Fig. 257 shows one form. They contain electromagnets so arranged that when the armature is attracted by the pushing of a button anywhere in the building, the door can be pushed open.

176. Dental Outfits. Fig. 258 shows a motor arranged to run dental apparatus. The motor can be connected to an ordinary incandescent light socket. In case the current gives out, the drills, etc., can be run by foot power.

177. Annunciators of various kinds are used in hotels, factories, etc., to indicate a certain room when a bell rings at the office. The bell indicates that some one has called, and the annunciator shows the location of the call by displaying the number of the room or its location. Fig. 259 shows a small annunciator. They contain electromagnets which are connected to push-buttons located in the building, and which bring the numbers into place as soon as the current passes through them.

INDEX.

Numbers refer to paragraphs. See Table of Contents for the titles of the various chapters.