Chapter IV

MAGNETS AND INDUCTION-COILS

Simple and Horseshoe Magnets

Every boy has a horseshoe magnet among his collection of useful odds and ends, and whether it is a large or small one its working principle is the same. If large enough it will lift a jack-knife, nails, or solid weights, such as a small flat-iron or an iron padlock. A horseshoe magnet is made of highly tempered steel and magnetized so that one end is a north pole and the other a south pole. In more scientific language these poles are known as, respectively, positive and negative. Once magnetized the instrument retains that property indefinitely, unless the power is drawn from it by exposure to intense heat, and even then, by successive heating and cooling, the magnetism may be partially restored.

An electro-magnet may be made of any scrap of soft iron, from a piece of ordinary telegraph-wire to a gigantic iron shaft. When a current of electricity passes through a wire a magnetic “field” is produced around the wire, and if the latter is insulated with a covering and coiled about a soft iron object, such as a nail, a bolt, or a rod, that object becomes a magnet so long as a current of electricity is passing through the coils of wire or helix. A coil of wire in the form of a spiral spring has a stronger field than a straight wire carrying the same current, for each turn or convolution adds its magnetic field to that of the other turns.

A simple form of electro-magnet is made by winding several layers of No. 20 insulated copper wire around a stout nail or a carriage-bolt; by connecting the ends to a battery of sufficient power, some very heavy objects may be lifted. A single magnet, like the one shown in Fig. 1, is made with a piece of soft iron rod six inches long and half an inch in diameter, the ends of a large spool sawed off and worked on the rod, and half a pound of No. 20 insulated copper wire. The spool-ends are arranged as shown in Fig. 2. An end of the wire is passed through a hole in one flange when you begin to wind the coils, and when finished, the other end is passed through a hole at the outer rim of the same flange. This magnet may be held in the hands when in use; or a hand-magnet may be constructed of a longer piece of iron on one end of which a handle is driven and held in place with a nut and washer, as shown in Fig. 3. The wires from the coil pass through holes made in the handle and come out at the butt end, where they may be attached by connectors to the pole-wires of a battery. To protect the outer insulated coil of wire from chafing and a possible short-circuit, it would be well to wrap several thicknesses of stout paper around the coil and glue it fast; or a leather cover will answer as well.

A more powerful magnet may be made from a stout bolt, two nuts, and a wooden base, with about three-quarters of a pound of No. 18 insulated copper wire to wind about the body of the bolt. A block of wood an inch thick, four inches wide, and six inches long is provided with a hole at the middle for the bolt to pass through. A larger hole is made at the under side of the block so that a nut can be easily turned in it. A three-quarter-inch machine-bolt, with a square head, and seven inches long, is set in the block, head up, as shown in Fig. 4; and composition or thin wooden disks or washers are placed on the bolt to hold the coils of wire in place. The ends of the wire pass out through the bottom washer and are made fast to binding-posts on the block, and to these latter the battery-poles are made fast when the magnet is in use. Coils of wire may be wound on an ordinary spool, and the hole in the middle may be filled with lengths of soft iron wire. When a current is passing around the spool the wires become highly magnetic, but lose the magnetism directly the current ceases.

Horseshoe electro-magnets are made by winding coils on the ends of [U]-shaped pieces of soft iron, but the winding must be done so that the current will pass around them in opposite directions, otherwise you would have two negatives instead of a negative and positive. For a small horseshoe magnet a stout iron staple may be used, but for the larger magnets it would be best to have a blacksmith bend a piece of round iron in the desired shape.

A powerful horseshoe magnet may be made from a piece of tire-iron bent as shown in Fig. 5 A; when wound with No. 18 wire it will appear like Fig. 5 B. A volt or two of current passing through the coils will render this magnet powerful enough to lift several pounds.

For bells, telegraph-sounders, and other electrical equipment requiring the horseshoe or double magnet, several kinds may be used, but the simplest is constructed from two carriage or machine bolts and a yoke of soft iron, as shown in Fig. 6. The yoke is five-eighths of an inch in width, two inches and a half long, and provided with two three-eighths-inch holes, one inch and a half apart from centre to centre. Two-inch carriage or machine bolts are used, and they should be three-eighths of an inch in diameter. The nuts are turned on the thread far enough to admit the yoke, and then another nut is applied to hold it in place and bind the three pieces into one compact mass. Wooden spool-ends or composition washers are placed on the bolts to hold the ends of the wire coils in place, and the winding may be done on each bolt separately and locked to the yoke after the winding is completed. Double cotton-insulated No. 20 or 22 copper wire should be used for the coils.

It is a tedious and bothersome job to wind a coil by hand, and if possible a winder should be employed for this purpose. Several varieties of winders are on the market, but a simple one for ordinary spools may be made from a stick held in an upright piece of wood with staples. This idea is pictured in Fig. 7, where the round stick is shown cut with two grooves into which the staples fit. One end of the stick is made with a square shoulder, so that a handle and crank can be fitted to it. A few wraps of wire are taken around the crank to prevent it from splitting, and it is held to the round stick with a slim steel nail. The opposite end of the round stick is shaved off so that it will fit snugly in the hole of a spool; if it should be too small for some spools, a few turns of cord around the small end will make it bind. The block to which the shaft and crank is attached may be held in a vise or screwed to the edge of a table.

Induction-coils

A simple induction or shocking coil may be made of a two-and-one-half by five-sixteenths-inch bolt, a thin wooden spool, and two sizes of insulated copper wire. An induction-coil is a peculiar and wonderful apparatus; it figures largely in electrical experimenting and is a part of every complete equipment.

A piece of curtain-pole may be used for the spool. First bore a five-sixteenths-inch hole through the wood to receive the bolt; then in a lathe turn it down into a spool with less than one-eighth of an inch of wood about the hole and with flanges about one-eighth of an inch in thickness. Smooth the spool with sand-paper, while it is still in the lathe, and give it a thin coat or two of shellac.

Slip the spool on the winder (Fig. 7) and wind on three layers of No. 24 cotton-insulated copper wire, taking care to wrap the coils evenly and close. Bring six inches of the ends out at either end of the spool through small holes pierced in the flanges; then wrap several thicknesses of brown paper around the coil. A current passing around this three-layer coil will magnetize the bolt. This is the primary coil and the one through which the battery current will pass.

A secondary coil is now made over the primary one with eleven or thirteen layers of No. 30 insulated copper wire. It will take some time to carefully put on these layers, and when doing so mark down each layer so as to keep an accurate count, for there must be the right number of layers to make the coil act properly. No. 30 wire is quite fine, and if the layers are not inclined to lie smooth, make a wrap or two of brown paper between each three layers. Bring six inches of each end of the wire out from the flanges of the spool, and to protect the outer coil wrap paper about the coils and attach it fast with thread or paraffine. Slip the bolt through the hole and screw the nut on the threaded end. Cut out a wooden block four inches long, three inches wide, and three-quarters of an inch thick, and with two thin metal straps and screws attach the coil to the middle of the block, as shown in Fig. 8. Make four binding-posts and screw them fast at the corners, and to A and B of Fig. 8 attach the ends of the heavy wire from the primary coil, and to C and D of Fig. 8 the ends of the fine wire from the secondary coils. The induction-coil is now ready for any use to which it may be put, and by mounting it on the block with the delicate wire ends attached to the binding-posts, it is in less danger of damage than if the wire ends were left loose for rough-and-ready connections.

In order to get a shock from this coil it will be necessary to have a pair of handles and a current interrupter. The handles may be made from two pieces of tin rolled into the form of cylinders to which wires are soldered. Or, better yet, use pieces of thin brass tubing an inch in diameter. The buzzer shown in Fig. 9 may be employed for a current interrupter, and a bichromate battery will furnish the current.

In order to make the connections, the wires from the handles are attached to the binding-posts C and D in Fig. 8--that is, to the wires of the secondary coil. One spool of the battery is connected with A of Fig. 8 and the other with A of Fig. 9. A wire connects C of Fig. 9 with B of Fig. 8, and the circuit is closed. The buzzer now begins to vibrate, and any one holding the handles will receive a shock the intensity of which depends on the strength of the batteries. A switch should be introduced somewhere in the circuit, so that it may be opened or closed at will; a good place for it is between a battery-pole and the binding-post A in Fig. 8.

If the shock is too intense it may be weakened by drawing the carbon and zinc poles partly out of the bichromate solution; or a regulator may be made of a large glass tube and a glass preserving-jar filled with water. If the tube cannot be had, an Argand gas-burner chimney will answer as well.

Solder a wire to the edge of a small tin or copper disk, as shown in Fig. 10, on which the chimney rests at the bottom of the jar, and another wire to a tin box-cover with some small holes punched in its top, this latter being suspended within the chimney. This second wire is passed out through a cork at the top of the chimney made of a disk of cardboard and a piece of wood. One wire is connected with A of Fig. 8 and the other with a battery-pole. This apparatus acts the same as a resistance-coil, and by raising or lowering the box-cover the current is increased or diminished. The closer the cover comes to the disk the stronger the current, as there is less water for the electricity to pass through and therefore less resistance; while if the cover touches the disk the current flows as freely as if there were no regulator and the wires ran directly to the cell.

An apparatus comprising a coil, an interrupter, or armature, and a switch may be set on one block, and the arrangement of the several parts is clearly shown in the drawing of the complete galvano-faradic apparatus (Fig. 11). The block should be six inches long, four inches wide, and seven-eighths of an inch in thickness.

The coil is made as described for Fig. 8, the spool being three inches long and one inch and a quarter in diameter. A carriage-bolt three inches and a half long and five-sixteenths of an inch in diameter, with a bevelled head, is made fast in the spool, and this coil is strapped to the block with two metal bands and screws. Two binding-posts (A and B of Fig. 11) are arranged at the upper corners, and to these the ends of the secondary coil wires are attached. Two more binding-posts (C and D of Fig. 11) are arranged at the lower side and provided with a switch to open and close the circuit. One of the primary coil wires is made fast to C, and the other one to a block which contains the set-screw that bears against the vibrating armature. Its arrangement and the wire connection is explained in Fig. 9 B.

An armature of thin brass or tin is made and attached to a block (E in Fig. 11). At the loose end that is opposite the bolt-head several wraps of tin are made and soldered fast, or a small block of soft iron may be riveted to the armature. It must be of iron or tin, however, so as to be attracted by the electro-magnetized bolt-head. This arrangement may be seen in Fig. 12. Attach a thick piece of paper over the bolt-head, so that the lug at the end of the armature will not adhere to it through residual magnetism.

In regular galvano-faradic machines the current regulator is formed of a hollow cylinder which is drawn from the core of the coil; but in this simple machine the water-jar regulator may be connected between a pole of the battery and the binding-posts (D or E of Fig. 11). The wires of the handles are attached to posts (A and B of Fig. 11), and when all the wires are in place and the current turned on by means of the switch, the vibrator begins to work and the shocking-current is felt through the handles. By means of the regulating-screw that bears on the armature, the number of vibrations may be increased or diminished, but for faradic purposes the vibrations should be as quick as possible. Much amusement may be had with this apparatus, and a large number of people may be given a shock by getting them to join hands when standing or sitting in a circle.

An Electric Buzzer

This piece of apparatus is, in theory, nothing more than the electric bell, and might properly be included in Chapter V., on Annunciators and Bells. But since it is the logical development of principles just laid down, it has been thought best to give it its present position.

The electric buzzer is constructed on the principle of the telegraph-sounder, but instead of making a single click or stroke the current is made to act on the armature and keep up a continuous motion so long as the electricity passes through the helix of the cores, the armature, and the contact-points of the apparatus.

A buzzer has the same movement as an electric bell with the ringing apparatus removed. For offices, houses, and quiet calls it is often preferred to the loud ringing of a bell.

The electric buzzer shown in Fig. 13 is easy to make; it is operated by the aid of a cell and a push-button. Cut a base-block three inches and a half wide, five inches long, and three-quarters of an inch thick, and mount a horseshoe magnet made of bolts and a yoke and coils about at the middle of it, as shown in Fig. 9. The magnet is held to the base by a flat wooden cleat and a screw passed down through a hole in the cleat and into the base, between the coils. An armature of soft iron, two inches long and half an inch wide, is riveted to a piece of spring-brass, as shown in Fig. 14 A, and the end is bent so that it will fit around the corner of a block to which it is held fast with two screws. This armature is mounted so that there is a space one-sixteenth of an inch wide between it and the bolt-heads, as you can see in Fig. 9. The brass is bent out slightly and runs parallel with the armature for one inch and a quarter. Against this the end of the screw mounted in block B Fig. 9 rests.

The block B is a small piece of hard-wood screwed fast to the side of the base to hold the set-screw and also the wire that comes from the outside of the upper coil. A small hole is made in the edge of the block and the wire passed in, so that the end rests in the screw-hole as shown by the dotted line. When the screw is placed in the hole and turned, it comes into contact with the wire and makes a connection. This block and its attachment is shown in Fig. 14 B.

On the base, near the armature-block, a binding-post is made fast, and the current, passing in through the wire A in Fig. 9, goes through the coils and around to the screw B, then through the armature to the block, and out through the wire C. In its circuit the bolts are magnetized, and they draw the armature, but the instant they do so the loose spring-brass end is pulled away from the screw-point B and the circuit is broken, the bolts cease to be magnetized, and the armature flies back under the influence of the spring-brass neck at D. The loose brass end, on touching the screw-point, conducts the current through the coils again, with a continual vibrating action, so long as the electric current is passing in at A and out at C. The greater the volume of current the greater the number of vibrations, and to properly regulate the contact the set-screw B must be adjusted at the right point. Paste pieces of heavy paper over the heads of the bolts to overcome residual magnetism.

A single electric bell is made the same as a buzzer, but continuing on from the end of the armature a wire or rod is mounted with a ball at the end which strikes the bell as the current causes the armature to vibrate. The bell-block may be made longer, and a bell from an old clock or a bicycle should be mounted at the proper place on a wooden dowel driven into the base. A screw passes through the hole at the middle of the bell and into the top of the dowel. The ball at the end of the rod may be made of brass with a hole in it, and a drop of solder will hold it in place. Or it may be made of wire wound round the end and soldered into a compact mass.

A Large Induction-coil

As has been said, the induction-coil is one of the mysterious phenomena of electrical science. While its practical value is known and recognized in all branches of voltaic electricity for use in transforming currents, its actual workings have never been clearly explained.

The construction of a small induction-coil was explained in the description of a shocker or medical battery. For bigger equipments, wireless telegraphy and other uses, a large induction-coil will be necessary, and the following illustrations and descriptions should enable the young electrician to construct an apparatus that will be both simple and efficient in its working.

For the tube (in which to wind the primary coils) obtain a piece of red fibre-tubing, one inch inside diameter and not more than one-eighth of an inch in thickness. The length should be ten inches. If fibre cannot be had use a paste-board tube.

From white-wood, half an inch in thickness, saw two blocks four inches square and in the centre of each cut a hole so that the tube will pass through it and fit snugly. Some shellac and a few slim brass escutcheon pins will hold the blocks in place, as shown at Fig. 15. The wood blocks and fibre or paper tube should be treated to several successive coats of shellac to give them a good finish and prevent the absorption of moisture. Four binding-posts, with wood screw-ends, are to be made fast at the top edges of the end-blocks, as shown at Fig. 15. Holes bored in the blocks near the foot of the binding-posts will admit the ends of the coil-wires so that contact can be made. The ends of the conductor-wires should then be placed in the holes in the binding-posts and held in place with the thumb-screws.

The primary coil is made by winding four layers of No. 20 insulated copper wire on the tube and between the end-blocks, as shown at Fig. 16. Each layer must be wound evenly, and the strands should lie close to each other. When the first layer is on give it a coat of shellac; then wrap a piece of thin paper about it and give that also a coat of shellac. When the second layer is on repeat the operation of shellacking and paper-coating, and continue with the third layer. When the fourth layer is on give the coil a double wrap of paper and two or three coats of shellac to thoroughly insulate it and keep out all moisture. The winding may be done by hand, but it is much easier to do it on a winder or reel, which can be operated to revolve the core, the wire unwinding from its original spool as it is wound on the tube.

A convenient winder may be made on a base-board which can be clamped to a table or bench. The board is twelve inches long, eight or ten inches wide, and seven-eighths of an inch thick. Two uprights, three inches wide, ten inches long, and three-quarters of an inch thick, are screwed and glued to the ends of the base-board. A notch is cut in the top of the end-boards, into which the spindle or shaft can rest; and at the top of the end-pieces two small plates of wood or metal are screwed down to hold the spindle in place when the tube and ends are being revolved. A small hole, bored in each upright end two inches above the top of the base-board, will admit a rod on which a spool of wire can revolve, as shown at Fig. 17.

Two plugs of wood, shaped like corks, are made to fit in the ends of the fibre-tube. A hole is bored through each one so that a wire or rod spindle will pass through them and fit tightly. One end of the rod is bent and provided with a small wooden handle, by means of which the core may be revolved.

This winding-rack makes it easy to handle the core-tube while putting on the layers of wire, and it holds the tube securely while the wraps of paper and shellac are applied.

The secondary coil is laid over the primary, and should be of Nos. 30 to 36 insulated copper wire. The finer the wire the higher the resistance and the longer the spark, but nothing heavier than No. 30 should be used.

Begin by making one end of the wire fast to a binding-post; then turn the core-tube with one hand, holding the wire in the other. Take care not to bind the wire nor stretch it, but wind it on smoothly and evenly, like the coils of thread on a new spool of cotton or silk. Be very careful to avoid kinks, breaks, or uninsulated places in the wire. Should the wire become broken, give the coil a coat of shellac to bind the wound strands; then make a fine twisted point and cover it with the silk or cotton covering, with a coat of shellac to hold it in place, and proceed with the winding. Between each layer of wire place a thin sheet of paper and coat it with paraffine, or shellac, to make a perfect insulation; then proceed with the next layer.

With a battery and small bell test the wire layers occasionally to see that everything is all right, and that there are no breaks or short circuits. This is very necessary to avoid making mistakes, and, considering the time and care spent in winding the coils, it would be a great disappointment if the coil were defective.

About one pound and a half of wire should constitute the secondary coil, and, if possible, it is best to have it in one continuous strand, without splices.

Over the last coil, after the winding is completed, several thicknesses of paper should be laid and well coated with shellac between each wrap. This is a protector to insure the fine wire strands from damage. To improve the appearance of the coil a wrap of thin black or colored leather may be glued fast, with the seam or point at the under side.

The ends of the wires forming the primary coil should be made fast to the binding-posts at one end, while those of the secondary coil should be attached to the posts at the other end.

For the core, obtain some soft iron wire, about No. 18, and cut a number of lengths. Straighten these short wires and fill the tube with them, packing it closely, so that the wires will remain in place under a mutual pressure. It is better to make a core of a number of rods or wires rather than to have it of one solid piece of soft iron.

Now, from hard-wood, cut a base three-quarters of an inch thick, five or six inches wide, and twelve inches long. Attach the coil to the base by means of screws passed up through the board and into the lower edges of the end-blocks. The wood is to be stained and given several successive coats of shellac.

Now connect the wires of a battery to the binding-posts in contact with the primary coil, and attach two separate wires to the secondary coil binding-posts. Bring these ends near to each other, and a spark will leap across from one end to the other, its size or “fatness” depending on the strength of the battery. The completed apparatus is shown at Fig. 18.

In producing a long spark a condenser is an important factor; it is used in series with an induction-coil. There are several forms of condensers, but perhaps the simplest and most efficient is the Fizeau condenser, which is made up of layers of tin-foil with paraffined paper as separators.

From a florist’s supply-house purchase one hundred and fifty sheets of tin-foil seven by nine inches, or sheets that will cut to that size without waste; also ten or twelve extra sheets for strips. At a paper supply-house obtain some clear, thin, tough paper about the thickness of good writing-paper. Be careful to reject any sheets that are perforated or have any fine holes in them. The sheets should be eight by ten inches, or half an inch larger all around than those of the tin-foil. The paper must be thoroughly soaked in hot paraffine to make it moisture-proof and a perfect non-conductor. This is done by placing about two hundred sheets on the bottom of a clean tin tray, or photographic developing-dish of porcelain. Don’t use glass or rubber. After placing some lumps of paraffine on the paper, put the tray in an oven so as to dissolve the paraffine and thoroughly soak the paper.

Open the oven door and, with a pin, raise up the sheets one at a time, and draw them out of the liquid paraffine. As soon as it comes in contact with cool air the paraffine solidifies and the sheet of paper becomes stiffened. Select each sheet with care, so that those employed for the condenser are free from holes or imperfect places.

From pine or white-wood, a quarter of an inch in thickness, cut two boards, eight by ten inches, and give them several good coats of shellac.

To build up the condenser, lay one board on a table and on it place two sheets of paraffined paper. On this lay a sheet of tin-foil, arranging it so that half an inch of paper will be visible around the margin. From the odd sheets of tin-foil cut some strips, one inch in width and three inches long. Place one of these strips at the left end of the first sheet of foil, as shown at Fig. 19. Over this lay a sheet of the paraffined paper, then another sheet of the foil. Now on this second sheet of foil lay the short strip to the right end, and so proceed until all the foil and paper is in place, arranging each alternate short strip at the opposite end. Care must be taken to observe this order if the condenser is to be of any use.

When the last piece of foil is laid on, with its short strip above it, add two or three thicknesses of paper, and then the other board. With four screw-clamps, one at each corner, press together the mass of foil, paper, and boards as closely as possible, then bind the boards about with adhesive tape, or stout twine, and release the clamps. Attach all the projecting ends of foil at one side by means of a binding-post, and those at the other end with another binding-post. The complete condenser will then appear as shown in Fig. 20.

When in operation one wire leading from the secondary coil should be connected with a binding-post of the condenser, so that it is in series.

The object of the condenser is to increase the efficiency of induction, and it should be made in proportion to the size of the induction-coil with which it is to be employed.

Circuit-Interrupters

When an induction-coil is to be employed as a shocker (and there is no vibrating armature arranged in connection with the core), a circuit-interrupter must be employed to get the effect of the pulsations, as given out by the secondary coil when a current is passing through the primary.

There are various forms of circuit-breakers that may be made for this purpose, but for really efficient service the type shown in Fig. 21 is perhaps the best that can be devised.

This interrupter consists of a metal cog-wheel with saw-teeth, a pinion or axle, and a handle. Also a base-block, with uprights to support it, and a piece of spring-brass wire, arranged so as to bear against the wheel. When the wheel is revolved the spring-wire will be driven out by each tooth; and when released it flies back to the wheel, striking the bevelled edge of a tooth at each trip.

Two binding-posts, arranged on the block, will provide means of connecting in-and-out wires. With a coat or two of shellac on the wood-work and black asphaltum varnish on all surfaces of the metal that are not used for contact, this circuit-interrupter will be ready for any use in connection with an induction-coil.

The base-block is of pine, white-wood, or cypress, seven-eighths of an inch thick, three inches wide, and five inches long. The uprights, which support the wheel, are half an inch thick and one inch wide. The wheel is three inches in diameter and is made of brass one-sixteenth of an inch thick. The design of the wheel should be laid out with a compass and marked with lead-pencil or a sharp-pointed awl, which will leave a mark clear enough to be seen when sawing and filing the teeth and open places.

A true plan is shown at Fig. 21 A. Through the middle of the wheel a small hole is bored to receive the axle of brass which is to be soldered in place. When the wheel is set up, a metal crank and wooden handle should be soldered fast to one end of the axle. A piece of spring-brass wire is fastened to the block, with a staple, and the lower end bent so that the screw in one binding-post will hold it in place. The upper end of the wire is bent in the form of an [L]. From the other binding-post, through the block and up one support, a wire is passed, the end of which comes into contact with the axle. The current, passing in through one binding-post, is carried through this wire to the axle, then to the wheel, and so on out through the spring-wire and remaining binding-post. When in action the circuit is constantly being broken, as the spring-wire jumps from the end of one tooth back to the face of the next tooth. The pulsations are increased or diminished by the fast or slow speed of the wheel, as regulated by the hand motion. The strength of the current is regulated by the force of the battery and should be controlled by a water resistance, as described for the medical battery, or shocking-coil.

The interrupter, shown in Fig. 22, is built up on a block six inches square and seven-eighths of an inch thick.

A circle is cut from sheet-lead and laid on the face of the block, through which pins, or steel-wire nails, are driven. The lead circle is five inches in diameter and half an inch in width, making the inside diameter four inches.

The pins or nails are driven a quarter of an inch apart, and should be properly and accurately separated, so that an even make-and-break will be the result.

It is not necessary to bore holes in the lead, but the pins or nails should be driven clear through it, so that perfect contact can be had by the metal parts coming together. Otherwise the apparatus would be useless.

Over the circle of pins a brass bridge is erected, so that the cross-strips will clear the heads of the pins. A hole is bored at the middle of the bridge so that the revolving axle will pass through it.

The axle is made from a piece of stout wire, or light rod, and near the foot of it, and about half an inch above the base-board, a disk of metal is soldered fast. A piece of spring-brass wire is attached to this disk, so that when the axle is turned the end of the wire trips from pin to pin, thus making and breaking the circuit. The upper part of the axle is bent and provided with a small wooden or porcelain knob.

One wire from the secondary coil is caught under a screw that holds one end of the brass bridge to the base; and the other to a screw, which may be placed at one corner of the block, and from which a short wire leads to the lead ring. Binding-posts may be arranged to serve the same purpose, and, of course, they are much better than the screws, because they can be easily operated by the fingers and do not require a screw-driver every time the interrupter is placed in series with an induction-coil. An interrupter on this same order may be made from a straight strip of lead with the pins driven through the middle of it. One wire from the secondary coil is made fast to the lead plate, and the end of the other wire is passed along the pins, thus making and breaking the circuit in a primitive manner.