Harper's Electricity Book for Boys

A simpler plunge-battery is shown in Fig. 10. A cell-rack is made of wood and given two or three coats of shellac. The narrow board (to the under side of which the battery-poles are attached, as explained in Fig. 9) is hung on chains or flexible wires, which in turn are made fast to an iron shaft running the entire length of the cell-rack. This shaft is of half-inch round iron, and is held in place, at one end, by a pin and washer; while at the other the end is filed with a square shoulder, and a handle and crank is fitted to it, so that the shaft may be turned. A small hole, made at the side of the crank when it is hanging down, will receive a hard-wood peg, or a steel nail, and this will prevent the crank from slipping when the board holding the poles is raised. If a gear-wheel and tongue can be had to fit on the shaft, it will then be possible to check the shaft securely at any part of a turn of the crank. The battery-poles are to be connected in series along the top of the portable board, as explained for Fig. 6. When two or more of these plunge-batteries are used at one time, the wire from the carbon of one is to be connected with the zinc pole of the next, and so on. The wire from the zinc of the first battery, and the wire from the carbon of the last battery, will be the ones available for use.

A Storage-battery

When more current is desired than the simple batteries will give, a storage-battery should be employed as an accumulator. This result can be secured by coupling primary cells in series, so that they will be constantly generating and feeding the battery. Storage-batteries are too heavy to be shifted about, like single cells or small plunge-batteries; they should be placed in a cellar, where the charging or primary cells can be located close by, and, unless positively necessary, the battery of cells and the accumulator should not be moved.

With sufficiently large insulated wires (Nos. 12, 14, or 16 copper), the current may be carried to any part of the house for use in various ways—such as running a light motor or a fan, lighting a lamp-circuit, or fusing metals and chemicals for experimental purposes. While the battery to be described is not a light one in weight, nor as economical as the improved new Edison storage-battery, it is a good and constant one, and, if not overcharged or abused, will last for several years.

The component parts of a storage-battery are lead in metallic and chemical form, the electrolyte, or fluid, in which the plates are immersed, and the water-tight and chemical-proof cell or container. From a plumber, a supply-house, or a lead-works, obtain a quantity of three-eighth by one-quarter-inch strip-lead of the kind called chemical, or desilverized; also a larger quantity of lead-tape, one-sixty-fourth of an inch thick and three-eighths of an inch wide. This last is also known as torpedo-lead, and is kept by electrical supply-houses.

If the three-eighths by quarter-inch strip-lead cannot be had, then purchase eight or ten pounds of heavy sheet-lead, and, with a tin-shears, divide it into strips three-eighths of an inch wide and twenty-nine inches long, taking care to cut it of uniform width and with true edges. From hard-wood three-eighths or half an inch thick, cut a block six by seven inches and make four countersunk holes in it, so that it may be screwed fast to a table or bench, as shown in Fig. 11 A. Around this the lead strips should be shaped and beaten at the corners to make the angles sharp.

From the three-eighths by quarter-inch, or sheet-lead strips, make seven frames as shown in Fig. 12. This is done by binding a strip of the lead around the block, as shown at Fig. 11 B. Where the ends come together insert a short piece of lead, three-eighths or half-inch, as shown at Fig. 12 A, and solder it fast. A soldering-iron may be heated with a Bunsen-burner gas-flame or in a charcoal fire. However, if gas is available, it would be better to use the blue flame from a Bunsen burner and direct the hot blast directly on the work with a blow-pipe, and so fuse the lead points together. After a little practice with the blow-pipe it will be used for many pieces of work in preference to the soldering-iron. If the sheet-lead is used for the frames in place of the three-eighths by quarter-inch strips, two or three strips will have to be taken, so as to build up the band of the frame to about a quarter of an inch in thickness. When soldered together, or fused at the edges, these built-up frames will be as rigid as the solid metal.

Now cut a number of strips of the thin lead-tape six inches and a half long, and others that will necessarily be somewhat longer, for each frame is to be filled with straight and crimped pieces, as shown in Fig. 13. If there is a fluting-iron in the house, the crimping may be done in the brass gears at one end of the machine. Or two wheels may be cut from hard-wood with a fret-saw, and made fast to a block with screws, as shown in Fig. 14. A handle, attached to one wheel, will make it possible to turn the gears; and they should be placed just far enough apart to allow the tape to pass through without tearing or squeezing. Put a washer between the wheel and the block to prevent friction.

When a frame is in the position shown in Fig. 13, and lying on a piece of slate or flat stone, you will first put in a crimped piece of tape, as shown at Fig. 13 A, and under this arrange a straight piece (Fig. 13 B); then, with the blow-pipe and flame, fuse fast to the frame and catch the flutes of the crimped piece to the straight one every inch or two. Add alternate crimped and straight strips until the frame is filled and presents the appearance of Fig. 13. When the seven frames are ready, lay three of them aside for the positives and four for the negatives. Note that the positives are red and the negatives a dark yellow when they are filled with the active material.

There are several methods of depositing the active material in the mesh or net-work of the plates, but some of them are too technical, others too complicated, and still others require charging machinery. The following plan will be the simplest and easiest for the amateur:

At a paint-store, or from a wholesale druggist, obtain several pounds of oxide of lead (red-lead) and a similar quantity of litharge (yellow-lead). In an earthen vessel, or large jar, make a solution composed of water, twenty ounces, and commercial sulphuric acid, two ounces. This is the mixture commonly known as “one to ten.” Place some red-lead (dry) in an old saucepan or soup-plate, and add a little of the acid solution: then, with an old table-knife or small trowel, mix the lead into a stiff paste, like soft putty. Do not get it too thin or it will run; nor too thick, as then it will not properly adhere to the lead-mesh of the frames. With the frame lying on its side, plaster in the red composition between the flutes and fill up the frame solid with it. Treat all three of the positive frames in the same manner, taking care that the exposed surfaces of the composition-filling is smooth and flush with the edges of the lead frame and mesh. Do not disturb these plates for a while, but let them remain in position, so as to set and partially dry. Add acid solution to the yellow-lead in a similar manner, and fill the four negative plates. When partially dry, the plates will be ready to combine in a pile.

At a supply-house obtain some sheets of cellulous fibre, three-sixteenths of an inch thick, or some asbestos cloth. If neither can be had, then soak some pieces of ordinary brown card-board in a solution of silicate of soda and let them dry. Lay a negative (yellow) plate on the table with the lug at the left (Fig. 13 C). On this place a square of the fibre, asbestos, or card-board; and on top of it lay a positive (red) plate with the lug at the right side. Continue in this manner until the seven plates are stacked, the four negative lugs being at the left and the three positives at the right. Tie the plates securely together with cotton string bound about them in both directions; then stand the pile up so that the lugs are at the top, as shown at Fig. 15, with every alternate lug in an opposite direction. Obtain two lead bars three-eighths of an inch square, or cut strips from the sheet-lead and solder them together, turning the ends as shown at Fig. 13 D. Drop one of these bars into the lugs of the positive plates, as shown in Fig. 15 H, and solder it fast at the three unions. Repeat this with the other bar in the lugs of the negative plates, and the pile will then be ready for immersion in the electrolyte. To both ends of each plate-bar solder binding-posts, so that the conductor-wires can be attached at one end and the feed-wires at the other. If a hard rubber or glass cell can be had for the battery so much the better; if not, a stout box may be made from pine, white-wood, or cypress, and thoroughly coated with asphaltum varnish or asphaltick. At an electrical supply-house you can purchase some “P and B” compound, which is acid and water proof. This is excellent for the inside coating as well as for the outside of the box.

The box should be made of wood not less than three-quarters of an inch thick, and the sides, ends, and bottom should be in one piece, free from knots, sappy places, or cracks. Brass screws should be used to hold the boards together, and before the joints are made the butt-ends of wood and the sides, against which they impinge, must be thoroughly coated with the asphaltum or compound. Put together the four sides first and then make the bottom fast, placing the screws two inches apart and countersinking the wood, so that the screw-heads will lie flush, as shown in Fig. 16. The box should be large enough to allow about one inch of space all around the pile, and deep enough for the solution to cover the plates and two inches of space above it to the top edge of the cell. The complete storage-battery will then appear as shown in Fig. 17.

The electrolyte is composed of sulphuric acid and water in the proportion of one ounce of acid to four of water, making a five-part solution. This should be mixed in an earthen or glass jar, and the acid poured slowly into the water, the latter being stirred while the acid is added. When the solution cools (for adding acid to water creates heat), add about two ounces of bicarbonate of soda, and mix the solution thoroughly.

When the pile is in place within the box (having first removed the string which bound the plates together) pour the electrolyte slowly into the cell, taking care that none of it spatters, for it will eat clothing or anything else that it touches. Before placing the pile, or electrolyte, in the box, it should be thoroughly tested for leaks by allowing water to stand in it for several days. Indeed, you should be very generous with the asphaltum, or compound, when coating the angles and points inside the box; for if the acid solution gets at the screws it will corrode them and the box will soon leak and fall apart. As a precaution against the acid working over the top of the box, the upper edge, for an inch or two, should be coated with paraffine over the asphaltum or acid-proof coating.

A cell constructed in this way should accumulate about two volts and one hundred ampere-hours, and will run a one-sixteenth horse-power motor. The expense of making these plates is about twenty-five cents each, and, including the cell and coating materials, each storage-battery will cost approximately two dollars. The lasting qualities of the battery depend on the use or abuse it is put to; but with ordinary care it should last from three to five years.

When the battery ceases to accumulate properly the pile should be removed, and, after washing it thoroughly, the bars should be cut away and new positive plates made and installed. The positive plates are the ones that deteriorate and need replacing; the negatives are almost everlasting, and with proper usage will live for fifteen or twenty years.

Directly the electrolyte is in the cell, connect the poles of your primary cells so as to begin the accumulation of current. Never exhaust the charge of electricity from your storage-cell, and never leave it uncharged when the electrolyte is in, or the plates will be ruined. A battery consisting of from five to twenty bluestone cells will be the best with which to charge this accumulator; and if more than one cell is desired, any number of them can be made and coupled up in series. Take care, when connecting the wires from the primary cells, to see that the positive wire is connected with the positive plates and the negative with the lead bar joining the yellow plates. If by accident you should make a misconnection, bubbles will rise from the electrolyte. This is not right, so reverse the wires and the accumulation of current will then take place without agitation in the cell.

Dry-cells and Batteries

Dry-cells are extensively used nowadays, since their cleanliness, high efficiency, and low internal resistance make them preferable to the Leclanché and other open-circuit batteries for bells, annunciators, and other light work. In the dry-cell, the electrolyte, instead of being a liquid, is a gelatinous or semi-solid mass, which will not run nor slop over. When the capping of pitch or tar is in place, the cell may be placed in any position, with full assurance that the electrolyte will not become displaced nor run out. Dry-cells may be made of almost any size for convenience of handling, but those commonly used vary from one to four inches in diameter, and from four to fifteen inches high. For bells and general electric work, a cell two inches and a half in diameter and seven inches high will be found a convenient size to make and handle.

The component parts of a dry-cell are the cell itself (which is made of zinc and acts as the positive pole), the carbon, the electrolyte or active excitant element, and the pitch or tar cap to hold the electrolyte and carbon in place.

From a tinsmith obtain some pieces of sheet zinc, and roll them into cylindrical form as shown in Fig. 18 A. The sheets should measure seven by eight inches, and when formed the edges are to be lapped and soldered.

From a smaller piece of zinc cut round bottoms, fit them in the cylinders and solder securely in place, taking care to close up all seams or joints to prevent the escape of the electrolyte.

From a supply-house obtain battery-carbons, one inch and a half wide by half or three-eighths of an inch thick and eight inches long. These should be provided with a thumb-screw or small bolt and nut at the top so as to make wire connections with the carbon. A strip of zinc should be soldered to the outside upper edge of the zinc cup to which wire attachments may be made with thumb-screws or small bolts and nuts. When the parts are ready to assemble, make a wooden mould or form a trifle larger than the carbon. This is intended to act as a temporary plunger, and is inserted, at first, in place of the carbon plate. This wooden plunger should be smooth, and given a coat of shellac to prevent it from absorbing any moisture.

Insert the plunger in the zinc cup and support it so that it will be at least half an inch above the bottom and centred at the middle of the cup. The electrolyte is then placed in the cup, and, when it has set a little, the wooden plunger is removed and the carbon inserted in its place.

The electrolyte is composed as follows:

Mix these together and place the compound within the zinc cups, so that the mass settles down and packs closely about the plunger. The space left unfilled about the carbon should be filled with a mixture composed as follows:

These proportions may be measured in a tin cup, a table-spoon, or any other small receptacle. Note that the measurement by parts is always by bulk and not by weight.

Do not fill the zinc cup to the top, but leave an inch of space, so that half an inch of sealing material may be added. See that the inside top edge of the zinc cup is clean; then melt some tar or pitch and pour it over the top of the electrolyte, so that it binds the zinc cup and carbon into a solid form. Drive an awl down through the capping material when it is nearly dry, and leave the holes open for the escapement of gases.

Give the outer surface of the zinc cells a coat of asphaltum varnish, and wrap several thicknesses of heavy paper about them to prevent contact and short-circuiting. Protect the bottoms in a similar manner, and as a result you will have a cell that will appear as shown in Fig. 18 B. A battery of cells powerful enough for any light work can be made by connecting the cells in series, each having an electro-motive force of one and a half volts, with an internal resistance of less than one-third of an ohm.

Chapter III
PUSH-BUTTONS AND SWITCHES

Push-buttons

Push-buttons and switches are a necessity in every home where electric bells, lights, or fans are used, for with them connections are made or broken. The telegraph-key and the commutators on a motor and dynamo are only improved forms of the push-button, and this simple little device is really an indispensable part of any electrical equipment.

The simplest form of push-button is a bent piece of tin or thin sheet-metal screwed fast to a small block of wood, as shown in Fig. 1. Under the screw-head one end of a wire is caught, and the other wire end is secured by a washer and a screw driven into the block directly under the projecting end of the strip of metal. By pressing a finger on the tin it is brought into contact with the screw-head under it, and the circuit is closed; on releasing it, the tin flies up and the circuit is opened again.

An enclosed push-button is shown in Fig. 2. It is made of the cover or body of a wooden box, a spool-end, and several other small parts. A round piece of thin wood is cut to fit inside the box and so form the base for the button. On this the spring strip is attached with screws, and the wire ends are made fast, as shown in Fig. 3. The wires are carried through the bottom of the base and along grooves to the edge, and thence to their final destination. The end of a spool is cut off and glued to the top of the box, as shown in Fig. 2, and a hole is made in the box to correspond in size with that in the spool. Through this aperture the button (cut from a wooden dowel or shaped out with a knife) passes, so that the end projects about a quarter of an inch beyond the spool. To prevent the button from falling out, a small steel nail should be driven across the inner end, or a washer may be tacked to the end of the stick, as shown in Fig. 4.

The button is mounted by screwing the base fast to the door or window casing, it being understood that the wires have been first arranged in place. The button is then set in the hole and the cap is placed over the base, covering it completely. By means of small screws, passed through the rim of the box and into the edge of the base, the cap is held in place. A coat of paint or varnish will finish the wood-work nicely, and this home-made button should then answer every requirement.

A simple switch (Fig. 5) is made from a base-block of wood three inches long, two wide, and half an inch in thickness, together with some small metal parts. It has but one contact-point, and that is the brass-headed tack (T in Fig. 5) driven through the binding-post, the latter being a small plate of brass, copper, or even tin screwed to the base-block. The end of a wire is caught under the screw-head before it is driven down. A similar binding-post is arranged at the lower side of the block, and the movable arm is attached to it with a screw. Between the arm and the post-plate there should be a small copper washer, to make it work more easily. The arm is cut from a thin piece of hard sheet brass or copper (tin or zinc will also answer very well), and at the loose end the half of a small spool is attached, with a brass screw and washer, to serve as a handle. The end of the screw that passes through a hole in the arm is riveted to the under side to hold it securely in place. This arrangement is shown in Fig. 6.

The under edges of the arm may be slightly bevelled with a file, so that it will slip up easily on the oval head of the brass tack. The drawing shows an open switch; when the circuit is closed the arm rests on the tack-head. By means of small screws this switch-board may be fastened to a table or to any part of the wood-work in a house.

In Fig. 7 a complex switch is shown. This is the principle of the shunt-box switch, of the resistance-coil, and also of the commutators of a motor. A motorman’s controller on a trolley-car is a good example of the shunt, and, with it and the resistance-coils, the car can be started, stopped, or run at any speed, according to the current that is admitted to the motor.

The complex switch is made in the same manner as described for the single switch, except that any number of binding-posts may be employed, arranged on a radial plan, so that the end of the arm will rest on any tack-head at will. Bells in various parts of the house may be rung by this switch, or it may be coupled with a series of resistance-coils to control any amount of current.

The simple cut-out (Fig. 8) is constructed in the same manner as the simple switch, except that there are two points of contact instead of one. This is the principle of the telephone and telegraph instrument wiring, so that a bell or sounder may be rung from a distance. The arm is then thrown over and the bell cut out, allowing the “phone” or key to be brought into use. In lifting the transmitter from the hook on a telephone, a cut-out is operated and the bell circuit is thrown out of action. It is in operation again directly the transmitter is returned to the hook. The switch cut-out (Fig. 9) is inactive when the arm is in the position shown in the illustration; but when it is thrown over (as shown by the dotted line) it connects the poles at opposite ends of the board. It may be thrown over in both directions, and is a useful switch for many purposes.

For strong currents the lever-switch, that rests on a brass tack-head, will not be suitable, as the switch-bar must be held firmly in place to make a perfect connection. Strong currents throw weak switches open, causing an open or broken circuit.

A single pole-switch, to carry a current up to one hundred and twenty-five volts and twenty-five amperes, is shown in Fig. 10. This consists of a base-block, a bar which is attached to the vertical ears of a binding-post, and a clutch that will hold the bar when it is pressed down between the ears.

The base-block should be made from some non-conducting material, such as soapstone, marble, or slate. If a piece of soapstone can be procured, that will be just the thing, since it is easily worked into the proper shape and size. Soapstone may be sawed and smoothed with a file; it is easily bored into with a gimlet-bit, and it is one of the best non-conducting substances. The base for this switch is six inches long, two inches wide, and as thick as the soapstone happens to be—say three-quarters of an inch. The top edge may be bevelled for the sake of appearance or left square.

Two pieces of heavy sheet copper or brass are to be cut as shown at A in Fig. 11. The ears are half an inch wide, and the total height of the strip is two inches and a half, while the part with two holes in it side by side is one inch and a quarter long, including the half-inch width of the vertical strip. With round and flat-nosed pliers bend the long ears into shape, so as to form a keeper for the bar which is then to be riveted in place. Omit the holes at the ends of the long ears in the other plate; then bend it into shape to form a clutch that will hold the bar when it is pressed down between the ears. These binding-posts should be made fast to the base-block with brass machine-screws and nuts, which will fit in countersunk holes in the bottom of the soapstone. If hard-wood is used for the base, ordinary brass wood-screws will answer very well.

The connection-bar is cut from metal the same thickness as that employed for the binding-posts and clutches; it should be shaped so as to appear as shown at B in Fig. 11. A handle should be driven on the slim end, and where the lower edge enters between the ears of the clutch, the corners of the bar should be rounded with a file. Countersunk screw-holes are bored in the base, so that it can be made fast to the wood-work.

A double pole-switch is shown in Fig. 12, and in general construction it is similar to the single pole-switch described above. The binding posts and bars are cut and bent from the patterns A and B in Fig. 11; but in this case the long, slim ends of the bars are omitted. A short turn is made at the handle end of each bar and a hard-wood block is placed between the bar-ends and held in position with screws driven through holes made in the bars and into the ends of the block. A handle is made fast to the middle of the block with a long and slim wood-screw; or a steel-wire nail may be passed through the handle and block, a burr slipped over the end opposite the head, and the small end riveted fast. When the binding-posts (to which the ends of the bars are attached) are screwed onto the base, be sure and see that the bars are parallel and the same distance apart at both ends. In like manner, when the cleat binding-posts are made fast, see that they are directly in line with the bars, so that the yoke will drop into the spaces between the ears without having to be pulled to one side or the other. This is a very useful switch for strong currents, and may be placed close to a dynamo, so that the current in both wires may be cut out at once.

Table-jack Switches

A table-jack switch is a most convenient piece of apparatus where several lines of bells, alarms, or telephone circuits are to be switched on and off.

The single table-jack switch, shown in Fig. 13, is made of a hard-wood block three-quarters of an inch thick, five inches wide, and seven inches long. It is to be smoothed and varnished, or given several coats of shellac. At the four corners small holes are made to receive slim screws, and at one end of the block five short metal plates are screwed fast, with the heads of the screws countersunk, so that they will be flush with the top of the plates. These small plates should be half an inch wide and one inch long, and may be of brass, copper, or tin. But if they are of tin the plates are made of a longer strip tacked to the board and then bent over, as shown at A in Fig. 14. They will therefore form short springs, the upper parts of which will rest against the long spring-arms. From spring brass or copper five arms are to be cut and shaped, as shown in Fig. 13. Holes are made at one end of each, and others again two inches from these, through which to pass screws.

Screw-eyes are passed through copper washers and the end holes in the strips, and then screwed into the wood plate. These will act as binding-posts, while the second line of screws will hold the plates down to the base. The arms should be bent, so that when the screws are driven down the lower edge will press on the small plates under them.

The outlet wires are attached to the binding-posts at the head of the block, and the plug (A in Fig. 13) is inserted between the arm and plate at the foot, so that contact and connection are made. This plug is a small plate of metal to which the end of a flexible wire is made fast. It should be of copper or brass, but for light work a strip of tin may be bent over with the wire caught between the plates and a copper tack passed through the sides and riveted, as shown at B in Fig. 14.

A double jack-switch (Fig. 15) is made on the same general plan as the single, but it has no binding-posts. A block of the same size is used, and two rows of short plates are made fast at each end. The arms are made with two screw-holes near the middle, as shown in Fig. 15, and through these holes screws are driven to hold the arms down to the base. Several plugs are used for each end, so that the in and out lines can be shifted, and from one to four lines used at a time.

A convenient slip-switch for single or double line work is shown in Fig. 16. This consists of a small wooden base, on which a brass arm and handle are screwed fast and connected with a binding-post (A in Fig. 16). A slip-plate is made from a piece of sheet-brass and bent so as to form a pocket into which the arm will fit. This pocket piece is connected with the binding-post B. When the switch is thrown out the circuit is broken, unless a contact-point, C, is provided, from the under side of which a wire leads out to a second circuit. When the switch is in place, as shown in Fig. 16, the circuit is closed through A and B; but when the arm is thrown out the circuit through A and B is broken and that through A and C is closed.

Binding-posts and Connectors

To make quick connections between wires and other parts of electrical apparatus, binding-posts are the most convenient device, since the turn of a screw binds or releases a wire instantly. Binding-posts may be made in many forms, but the simple ones that a boy will need can be made from screw-eyes, burrs, stove-bolts, and nuts, together with thin strips of metal and nails.

Five simple posts are shown in Fig. 17. A is made from a screw and two burrs, B from a screw-eye and two burrs, and C from a thin plate of metal and two screws, with oval or round heads. This last, however is more of a connector than a binding-post. The ends of the wires to be connected should be caught under the screw-heads or between the burrs before the screws are driven down.

In D a simple arrangement of a stove-bolt and two nuts is shown. The under bolt is screwed down tightly against the wood, and under the head a wire is made fast, so that another wire may be caught under the upper nut. If a small thumb-nut can be had in place of the plain nut, it will be easier to bind the upper wire. In Fig. 17 E a thin strip of metal may be folded over, and at the loose ends a hole should be punched through which a screw-eye will pass. The metal is held to a wood base with a screw, under the head of which a wire is caught. The second wire end is slipped between the metal plates, and a turn of the screw-eye will bind and hold it securely.

Connectors are employed to unite the ends of wires temporarily, and are made in many forms. A simple and useful one is made from a piece of spiral spring fastened to a block of wood by two staples, as shown at Fig. 18 A. The ends of the wires are pressed down into the coils of the spring and are held with sufficient security for temporary use. Another connector is made from a block of wood, a strip of thin metal, and two screw-eyes (Fig. 18 B). The metal is bent around the ends of the block, and through holes made in the ends of the strip screw-eyes are driven into the block. When the ends of wires are slipped under the metal, a turn of the eyes will hold them fast, as shown at Fig. 18 B.

A short bolt threaded at each end and provided with four nuts will also act as a connector. The inner nuts are screwed on tightly and the outer ones are loose, so that when wires are placed between them they may be tightened with the fingers, as shown at C in Fig. 18. These are a few simple forms of connectors; the ingenious boy can devise many others to suit his needs and ideas.

Lightning-arresters and Fuse-blocks

All lines of exposed wire that run from out-doors into the house should be provided at both ends with lightning-arresters, particularly if they are telephone or telegraph lines, burglar alarms, or messenger call-boxes. In many instances where unprotected telephone lines have been the plaything of lightning, painful accidents have happened, and it is only the part of prudence to provide against them. It is better to have an arrester at both ends of a line, and as the cost is insignificant it is hardly worth considering as against its feature of safety.

Lightning-arresters may be constructed in many ways and of different materials; the ones here shown and described are easily made and efficient. The principle of all arresters is simply a fuse which burns out whenever the wire is carrying a greater amount of current than is required for the proper working of the apparatus, thereby arresting the current and protecting the instruments from destruction. Induction-coils, relays, fine windings on armatures, or a magnet helix are quickly destroyed if a too powerful current is permitted to pass through them, and it is therefore advisable to protect them. When a fuse burns out under a trolley-car, or in the shunt-box of a motor-car or engine, it is because a greater amount of current is trying to pass in than the motor will safely stand. When a fuse “blows out,” the apparatus or motor is put out of commission until the fuse is replaced, but the delicate mechanism and the fine wiring on the field-magnets or armatures are saved.

The simplest form of single pole-fuse is a fine piece of lead wire held between two binding-posts, as shown at A in Fig. 19. The lead wire may be of any length; but for small instruments, where a moderate current is employed and where there is a possibility of lightning travelling on the wire, the fuse should be from two to three inches long. For inside work, however, where it is to be used simply as a safety, the wire may be shorter and finer.

To make the lightning-arrester shown in Fig. 19, cut out a hard-wood block five inches long, an inch wide, and half an inch thick. Give this several coats of shellac; then place a piece of mica, or asbestos paper, over the top of the block, and make it fast with thick shellac to act as a glue. From small pieces of copper or brass cut two plates one-half by one inch, and drill holes in them to take screws and screw-eyes. Place copper burrs under the screw-eyes for connectors, and drive two brass screws half-way down in the block through the holes at the inner ends of the binding-post plates. See that these screws fit snugly in the holes in the plates so that contact is perfect. If the holes are too large and the screws fit loosely, two copper burrs will have to be used and the screws driven in, so that the heads bind the burrs on the ends of the fuse-wire. From an electrician, or supply-house, purchase a few inches of fine lead fuse-wire—say Nos. 20, 22, or 24—and twist the ends of a length around the screws, as shown in the drawing. Perfect contact should be had between the lead wire and the screws; by way of precaution, a bit of solder will dispel all doubt. Just touch the point with a little soldering solution; then apply a soldering-iron having a drop or two of solder on the end.

Perfect connection is absolutely necessary for telephone, telegraph, or annunciator work, and where there is a lightning-arrester and the line is not working well, the trouble may often lie in the poor contact of lead and brass or copper, or possibly in using wire that is too fine. Lead is a very poor conductor, and a fine wire would act as a check. For a test, first insert a piece of copper wire to see that the line is working properly; then use lead wire of sufficient size to carry the current as well as the copper did. The action of metals and wire, as current retarders, will be explained in the chapter on resistance and resistance-coils.

For general commercial use the base-blocks of all lightning-arresters should be made of porcelain, slate, or some of the composition non-conductors, such as moulded mica, silex and shellac, or fibre. As these are not always available, wood, with a covering of mica, will answer every purpose and can be readily adapted for use.

The apparatus pictured in Fig. 19 is known as a single-pole lightning-arrester, and is the simplest form of this kind of electrical paraphernalia. In Fig. 20 a double-pole arrester is shown. This is constructed in the same manner as described for the single one. The block is five inches long, two inches wide, and half or five-eighths of an inch thick. A countersunk hole is made in the middle of all the lightning-arrester blocks through which a screw can be passed to hold the apparatus fast in any desired location.

In Fig. 21 another form of fuse is shown. It is made from a piece of mica three-quarters of an inch wide and four inches long, two pieces of thin sheet-copper, and a piece of lead fuse-wire. The copper is three-quarters of an inch wide, and one piece of it is bent in the form of a V, as shown at A in Fig. 21. One end of the mica strip is dropped in the V, and with a pair of pliers the V is closed up by pinching it at the bottom. To further insure its staying in place, the top and end, or open edges, should be soldered. Punch a small hole through the copper ends, at the inside edge, slip the ends of the fuse-wire in them, and touch the union with a drop of solder to insure perfect contact. With shears and file cut a U from the side of one copper band and from the end of the other; these will allow the copper ends to pass under the heads of screws, thus avoiding the necessity of removing the entire screw from the block in order to set the fuse in place.