This style of resistance equipment is the least expensive to make, and will give excellent satisfaction to the boy who is using light currents for induction-coils, lamps, galvanometers, and testing in general. The simplest form of liquid resistance is made by using a glass bottle with the upper part cut away. The cutting may be done with a steel-wheel glass-cutter. The bottle should then be tapped on the cut line until the top part falls away. Go over the sharp edges with an old file to chafe the edge and round it; then solder a tin, copper, or brass disk to a piece of well-insulated wire and drop it down in the bottom of the receptacle, as shown at Fig. 17. Cut a smaller disk of metal, or use a brass button, and suspend it on a copper wire which passes through a small hole in a piece of wood at the top of the jar. Notches should be cut at the under side of this wood cross-piece so that it will fit on top of the jar and stay in place. The jar is to be nearly filled with water, having a teaspoonful of sulphate of copper dissolved in it. This will turn the water a bluish color and make it a slightly better conductor, particularly when the button is lowered close to the round disk. If a high resistance is desired the copper may be omitted leaving the water in its pure state. The wires leading in and out of the jar should be connected between the apparatus and the battery so that the proper amperage can be had by raising or lowering the button. A series of these liquid resistance-jars may be made of glass tubes an inch in diameter and twelve inches long. One end of them may be stopped with a cement made of plaster of Paris six parts, ground silex or fine white sand two parts, and dextrine two parts. Mix the ingredients together when dry, taking care to break all small lumps in the dextrine; then add water until it is of a thick consistency like soft putty. Solder the ends of some copper wires to disks of copper or brass and set them on the middle of bone-buttons; these in turn are to be imbedded in the mixture after the wire has been passed through a hole in the bottom.
Their location can be seen in the bottom of the tubes Fig. 18, and Fig. 19 A is an enlarged figure drawing of the plate, button, and wire. The wires are brought out under the lower edge of the tubes, and enough of the composition is floated about the bottom and outer edge of the tube to form a base, as shown in the drawing. A base-board is made six inches wide and long enough to accommodate the desired number of tubes. Two pieces of wood one inch wide and three-quarters of an inch thick have hollow notches cut from them at one side, as shown at Fig. 19 B. In these notches the tubes are gripped. Screws are passed through one stick and into the other so as to clamp the wood and tubes securely together. The rear stick is supported on two uprights which are made fast to the rear edge of the base-plate with screws and glue.
Along the front of the base-board small metal contact plates, or binding-posts, are arranged (see Binding-posts, chapter iii.) and the wires led to them from the tubes, as shown in the drawing. The top or drop wires in the tubes are provided with metal buttons at the ends; or the end of the wire may be rolled up so as to form a little knob. The manner of connecting the wires was freely explained in the resistance-coil descriptions and may be studied out by examining the drawing closely. In this resistance-apparatus there are two ways of cutting out a medium—first, by lowering the wire in the tube so that both contact-points meet; and second, by cutting out the first tube altogether by connecting the incoming wire with the second binding-post. Then again the resistance may be regulated quite accurately by raising or lowering the wires in the liquid.
For example, there is too much resistance if the current has to travel through all the tubes. If it is too strong when one tube is cut out, the wire in tube No. 1 is lowered so that the contacts are an inch apart. Then the more accurate adjustment is made by dropping the wire in the second tube, as shown in Fig. 18. The wires leading out at the top of the tubes are pinched over the edge to hold them in place. They should be cotton insulated and the part that is in the liquid should be coated with hot paraffine.
The water may be made a slightly better conductor if a small portion of sulphate of zinc, or sulphate of copper, is added to each tubeful.
Hittorf’s resistance-tube is one of the oldest on these lines, and two or more of them are coupled in series, as described for this water-tube resistance; glass tubes are employed that have one end sealed with a permanent composition, as described for Fig. 18. A metallic cadmium electrode is placed at the bottom of the tube, and the tube is then filled with a solution of cadmium iodide one part and amylic alcohol nine parts, and then corked. A wire passing down through or at the side of the cork is attached to another small piece of metallic cadmium, which touches the top of or is suspended a short distance in the liquid.
As the alcohol is volatile the cork cannot be left out of the tube, and the wire must be drawn through the cork with a needle so that no opening is left for evaporation. A number of these tubes may be made and coupled in series and the wires led down to the contact-points of a switch.
Chapter VIII
THE TELEPHONE
For direct communication over short or moderately long distances, nothing has been invented as yet that will take the place of the telephone. A few years ago, when this instrument was first brought out, it was the wonder of the times, just as wireless telegraphy is to-day. Starting with the simple form of the two cups with membranes across the ends, and a string or a wire connecting them, we have to-day the complex and wonderful electric telephone, giving perfect service up to a distance of two thousand miles. Some day inventors in the science of telephony will make it possible to communicate across or under the oceans, and when the boys of to-day grow to manhood they should be able to transact business by ’phone from San Francisco to the Far East, or from the cities near the Atlantic coast to London, Paris, or Berlin.
It is hardly necessary to enter into the history of telephones, as this information may be readily found in any modern encyclopædia or reference work. But the boy who is interested in electricity wants to know how to make a telephone, and how to do it in the up-to-date way, with the wire and ground lines, switches, cut-outs, bell connections, and other vital parts, properly constructed and assembled. In this laudable ambition we will endeavor to help him.
The general principle of the telephone may be explained in the statement that it is an apparatus for the conveyance of the human voice, or indeed any sounds which are the direct result of vibration.
Sound is due to the vibrations of matter. A piano string produces sound because of its vibration when struck, or pulled to one side and then released. This vibration sets the air in rapid motion, and the result is the recording of the sound on our ear-drums, the latter corresponding to the film of sheepskin or bladder drawn over the hollow cup or cylinder of a toy telephone. When the head of a drum is struck with a small stick it vibrates. In this case the vibrations are set in motion by the blow, while in the telephone a similar phenomenon is the result of vibratory waves falling from the voice on the thin membrane, or disk of metal, in the transmitter. When these vibrations reach the ear-drum the nervous system, corresponding to electricity in the mechanical telephone, carries this sound to our brains, where it is recorded and understood. In the telephone the wire, charged with electricity, carries the sound from one place to another, through the agencies of magnetism and vibration.
Over short distances, however, magnetism and electricity need not be employed for the transmission of sound. A short-line telephone may be built on purely vibratory principles. Almost every boy has made a “phone” with two tomato-cans over which a membrane is drawn at one end and tied. The middle of the membrane is punctured, and a button, or other small, flat object, is arranged in connection with the wires that lead from can to can.
A Bladder Telephone
A really practical talking apparatus of this simple nature may be made from two fresh beef bladders obtained from a slaughter-house or from the butcher. You will also need two boards with holes cut in them, two buttons, some tacks, and a length of fine, hard, brass, copper, or tinned iron wire. The size should be No. 22 or No. 24. The boards should be ten by fourteen inches and half an inch in thickness. Cut holes in them eight inches in diameter, having first struck a circle with a compass. This may be done with a keyhole saw and the edges sand-papered to remove rough places. Prepare the bladders by blowing them up and tieing them. Leave them inflated for a day or two until they have stretched, but do not let them get hard or dry.
When the bladders are ready, cut off the necks, and also remove about one-third of the material, measuring from end to end. Soak the bladders in warm water until they become soft and white. Stretch them, loosely but evenly, over the opening in the boards, letting the inside of the bladder be on top, and tack them temporarily all around, one inch from the edge of the opening. Test for evenness by pushing down the bladder at the middle. If it stretches smoothly and without wrinkles it will do; otherwise the position and tacks must be changed until it sets perfectly smooth.
The bladder must now be permanently fastened to the board by means of a leather band half an inch wide and tacks driven closely, as shown in Fig. 1. With a sharp knife trim away the rough edges of the bladder that extend beyond the circle of leather. Attach a piece of the fine wire to a button, as shown in Fig. 2, and pass the free end through the centre of the bladder until the button rests on its surface. Then fasten an eight-pound weight to the end of the wire and set in the sun for a few hours, until thoroughly dry, as shown at Fig. 3.
When both drums are complete, place one at each end of a line, and connect the short wires with the long wire, drawing the latter quite taut. The course of the main wire should be as straight as possible, and should it be too long it may be supported by string loops fastened to the limbs of trees, or suspended from the cross-piece of supports made in the form of a gallows-tree or letter F. To communicate it will be necessary to tap on the button with a lead-pencil or small hard-wood stick. The vibration will be heard at the other end of the line and will attract attention.
By speaking close to the bladder in a clear, distinct tone, the sound will carry for at least a quarter of a mile, and the return vibrations of the voice at the other end of the line can be clearly recognized.
A Single (Receiver) Line
The principal parts of the modern telephone apparatus are the transmitter, receiver, induction-coil, signal-bell, push-button, batteries, and switch. The boxes, wall-plates, etc., etc., are but accessories to which the active parts are attached.
The first telephone that came into general use was the invention of Graham Bell, and the principle of his receiver has not been materially changed from that day to this, except that now a double-pole magnet and two fine wire coils are employed in place of the single magnet and one coil. A practical form of single magnet receiver that any boy can easily construct is shown in Fig. 4, and Fig. 5 is a sectional drawing of the receiver drawn as though it had been sliced or sawed in two, from front to rear.
It is made from a piece of curtain-pole one inch and an eighth in diameter and three inches and a half long. A hole three-eighths of an inch in diameter is bored its entire length at the middle, and through this the magnet passes. At one end of this tube a wooden pill-box (E) is made fast with glue, or a wooden cup may be turned out on a lathe and attached to the magnet tube. If the pill-box is employed it should be two inches and a half in diameter, and at four equidistant places inside the box small lugs of wood are to be glued fast. Into these lugs the screws employed to hold the cap are driven. The walls of pill-boxes are so thin that without these lugs the cap could not be fastened over the thin disk of metal (D) unless it were tied or wired on, and that would not look well. If the cup is turned the walls should be left thick enough to pass the screws into, and the inside diameter should then be one inch and three-quarters.
The cap (B) is made from thin wood, fibre, or hard rubber. It is provided with a thin rim or collar to separate its inner side from the face of the disk (D). Four small holes are bored near the edge of this cap, so that the screws which hold it fast to the cup (E) may pass through them. The magnet (M) is a piece of hard steel three-eighths of an inch in diameter and four inches and a quarter long. This may be purchased at a supply-house, and if it is not hard enough a blacksmith can make it so by heating and plunging it in cold water several times. It may be magnetized by rubbing it over the surface of a large horseshoe magnet, or if you live near a power station you can get one of the workmen to magnetize it for you at a trifling cost. Should you happen to possess a bar magnet of soft iron with a number of coils of wire, and also a storage-battery, the steel bar may be substituted for the soft iron core and the current turned on. After five minutes the steel can be withdrawn. It is now a magnet, and will hold its magnetism indefinitely.
Now have a thin, flat spool turned from maple or boxwood to fit over one end of the rod, and wind it with a number of layers of No. 36 copper wire insulated with silk. This is known in the electrical supply-houses as “phone”-receiver insulated wire, and will cost about fifty cents an ounce. One ounce will be enough for two receivers. It should be wound evenly and smoothly, like the strands of thread on a spool, and this may be done with the aid of the winder described on page 58.
When the wire is in place a drop of hot paraffine will hold the end so that the wire will not unwind. The ends of this spool-winding should be made fast to heavier wires, which are run through small holes in the tube (A) and project out at the end, as shown at F F. The magnet, with its wire-wound spool on the end, is then pushed through the hole in A until the top end of the rod is slightly below the edges of the cup (E), so that when the metal disk (D) is laid over the cup (E) the space between the magnet and disk, or diaphragm (D), is one-sixteenth of an inch (see Fig. 5). Put some shellac on the magnet, so that when it is in the right place the shellac will dry and hold it fast.
The cap (B) holds the disk (D) in place, and protects the spool and its fine wire from being damaged and from collecting dust. After giving the exterior a coat of black paint and a finishing coat or two of shellac, the receiver will be ready for use.
The original telephone apparatus was made up of these receivers only—one at each of a line in connection with a battery, bell, push-button, and switch. On a window-casing, or the wall through which the wires passed, a lightning-arrester was arranged and made fast. Using receivers only, it was necessary to speak through the same instrument that one heard through, and for a few years this unhandy method of communication was the only one possible. Then the transmitter was invented.
Plan of Installation
Many of these single-receiver lines are still in use, and as they require but a small amount of constructive skill a diagram of the wiring and the plan of arrangement is shown in Fig. 6.
At the left side, R is the receiver at one end of the line and R 2 that at the other, line No. 1 being a continuous wire between the two receivers. When the boy at R wishes to call his friend at R 2 he uses his push-button (P B), and the battery (B B) operates the electric bell (E B 2) at the other end. In order to have the bell connections operative, the switch (S 2) must be thrown over to the left when the line is “quiet,” while the switch (S) should be thrown to the right. With the switches in this position the boy at either end may call his friend at the opposite end.
With the switch (S 2) thrown to the left (the position it should be in, except when talking over the line), the boy at the other end pushes his button (P B), first throwing switch S to the left. This makes connection for the battery (B B), and the circuit is closed through wires that join line No. 1 and line No. 2 at 1 and 2. The branch lines to the bell (E B 2) join the main lines at 3 and 4, through switch S 2, when the bar is thrown to the left. The circuit being complete, the batteries (B B) at one end of the line ring the bell (E B 2) at the other end of the line.
In the reverse manner, when the switch (S) is thrown to the right, the boy at the opposite end rings the bell (E B) by pressing on the button (P B 2), first throwing switch S 2 over to the right. If the boy at the left is calling up the boy at the right, the switch (S) should be thrown to the left, and he keeps ringing until the other operator throws switch S 2 over to the right. If now he has the receiver (R) up to his ear he can hear the vibration of the bell (E B 2) ringing through the receiver (R) at his end of the line. But when the boy summoned to R 2 takes up the receiver and places it to his ear, he throws switch S 2 over to the right side, and the boy at R leaves switch S over on the left side. This brings the lines into direct connection with the receivers in series. Be careful, when setting up this line, to have the batteries (B B) in series with B 2 B 2; otherwise there would be counter-action. The carbon of one cell should be connected with the zinc of the next cell, and so on.
Another receiver is shown at Fig. 7. The tube (A) and the cup are turned from one piece of wood, and the cap (B) from another piece. The length of the receiver is five inches, and the cap is two inches and a half across. The shank, or handle, through which the magnet is passed measures one inch and a quarter in diameter.
These wood parts will have to be made by a wood-turner; and before the long piece is put in a lathe the hole, three-eighths of an inch in diameter, should be bored. It must be done carefully, so that the wood shell will be of even thickness all around the hole. Also two small holes should be made the entire length of the handle, through which the wires leading from the coil to the binding-posts may pass.
The spool for the fine insulated wire coil is turned from box-wood or maple, and wound as described in chapter iv., on Magnets and Induction-coils. Small binding-posts (F F) with screw ends should be driven down into the holes at the end of the handle and over the bare ends of the wires that project out of the holes. The magnet (M) is three-eighths of an inch in diameter, and is provided with the spool and coil (C) at the large end of the receiver.
The disk (D) is of very thin iron, and is held in place by the cap (B) and four small brass screws driven through the edge of B and into the cup end of A. A screw-eye should be driven into the small end of the receiver from which it may hang from a hook. If a double hook and bar is employed the receiver will hang in the fork, being held there by the rim of wood turned at the small end of A.
A Double-pole Receiver
Another form of receiver is shown at Fig. 8. This is a double-pole receiver, with the coils of fine wire arranged on the ends of a bent band of steel and located in the cup (A), so that the ends of the magnet are close to the diaphragm (D). Fig. 8 is a sectional view of an assembled receiver, but a good idea can be had from the drawings of the separate parts. The magnet (M) is of steel one-eighth of an inch thick and five-eighths of an inch wide. A blacksmith will make this at a small cost. It should measure two and one-half inches wide, two and one-half inches long, the ends being five-eighths of an inch apart.
Thin wooden spools are made from wood or fibre to fit over the steel ends, and are wound with No. 36 silk-insulated wire. A wooden cup, or shell (A), is turned from cherry, maple, or other close-grained wood, and at the back a hole is cut just large enough for the magnet ends to slip through exclusive of the coils wound on them. A plug of wood (A A) is driven between the ends of the magnet to hold them in place. Some shellac on the edges of the hole and the plug will harden and keep the parts in place.
The coils (C C) are placed on the magnet ends, and the fine wires are made fast to the binding-posts (E E), the latter being screwed fast to the shell (A). The diaphragm (D) is then arranged in place and held with the cap (B) and the small screws which pass through it and into the shell (A).
The Transmitter
With any one of these receivers a more complete and convenient telephone can be made by the addition of a transmitter and an induction-coil.
Following the invention of the receiver, several transmitters were designed and patented, among them being the Edison, Blake, Clamond, Western Union, and Hunning. The Edison and Hunning are the ones in general use, and as either of them can easily be made by a boy a simplified type of both is shown in Figs. 9 and 11.
Some small blocks of wood, tin funnels, small screws, granulated or powdered carbon, some thin pieces of flat carbon, and a piece of very thin ferrotype plate will be the principal things needed in making a transmitter similar to the one shown in Fig. 9. All that is visible from the outside is a plate of wood screwed to a block of wood, and a mouth-piece made fast to the thin board.
In Fig. 10 an interior section is shown, which when once understood will be found extremely simple. The block (A) is of pine, white-wood, birch, or cherry, and is two inches and three-quarters square and five-eighths or three-quarters of an inch thick. A hole seven-eighths of an inch in diameter is bored in the centre of this block, half an inch deep, and a path is cut at the face of the block one inch and a half in diameter and one-eighth of an inch deep. Be careful to cut these holes accurately and smoothly, and if it is not possible to do so, it would be well to have them put in a lathe and turned out.
The face-plate (B) is two inches square, with a three-quarter-inch hole in it, and the under-side is cut away for one-eighth of an inch in depth and one inch and a half in diameter. The object of these depressions in block A and face-plate B is to give space for the diaphragm (D) to vibrate when the voice falls on it through the mouth-piece (C).
From carbon one-eighth of an inch in thickness two round buttons are cut measuring three-quarters of an inch across. A small hole is bored in the centre of each button, and one of them is provided with a very small brass screw and nut, as shown at F F. One side of the button-hole is countersunk, so that the head of the screw will fit down into it and be flush with the face of the carbon. With a small three-cornered or square file cut the surface of the buttons with criss-cross lines, as shown at F F. When the buttons are mounted in the receiver these surfaces will face each other. Cut a small washer from felt or flannel, and place it in the bottom of the hole in block A. Line the side of the hole with a narrow strip of the same goods; then place the button (F F) in the hole, pass the screw through the hole and through the block (A), and make it fast with the nut, as shown at F. Place a thin, flat washer under the nut, and twist a fine piece of insulated copper wire between washer and nut for terminal connections, taking care that the end of the wire under the nut is bare and bright, so that perfect contact is assured. Since the practice of telephony involves such delicate and sensitive vibratory and electrical phenomena, it is best to solder all joints and unions wherever practicable, and so avoid the possibility of loose connections or corrosion of united wires.
From very thin ferrotype plate cut a piece two inches square, and at the middle of it attach the other carbon button by means of a small rivet which you can make from a piece of copper wire. Or a very small brass machine screw may be passed through the button and plate; then gently tapped at the face of the plate to rivet it fast, as shown at E. Lay the block down flat and partly fill the cavity with carbon granules until the button is covered. Do not fill up to the top of the hole. Over this lay the disk (D), so that the carbon button at the under side of it will fit in the top part of the hole between the sides of felt or flannel. Make the disk fast to the block (A) with small pins made by clipping ordinary pins in half and filing the ends.
A slim bolt (G) is passed through the block (A), and a wire terminal is caught under a nut and between a washer at the back of the block, as described for F. The japan or lacquer must be scraped away from the disk (D) where the bolt-head touches it, so that perfect electrical contact will be the result.
A small tin funnel is cut and made fast to the face-plate (B), or if an electrical supply-house is at hand a mouth-piece of hard rubber or composition may be had for a few cents. The block (B) is then screwed fast to A, forming the transmitter shown at Fig. 9. When this transmitter stands in a vertical position the granules, or small particles of carbon, drop down between the buttons of carbon, packing closely at the bottom of the cavity. At the middle they are loosely placed, and at the top there are none. As the high or low vibrations of the voice fall on the disk (D) they act accordingly on the carbon granules, which in turn conduct the vibrations to the rear carbon button, and, by the aid of electricity reproduce the same sound, in high or low tone, through the receiver at the other end of a line.
This improved transmitter makes it possible to talk in a moderate tone of voice over distances up to one thousand miles, while with the old form of the instrument it was necessary to talk very loud in order to be heard only a few miles away. Where a portable apparatus is desired, this block may be attached to a box or an upright staff.
This transmitter will not work when on its back or so that the funnel is on top, because the particles of carbon would settle on the rear button and not touch the front one. It is essential that the carbon grains should touch both buttons at the same time, and at the lower part of the cavity they should lie quite solid. It is not necessary, however, to pack it in, for the vibratory action of the voice, or other sounds, will cause the particles to adjust themselves and settle in a compact mass.
Another Form of Transmitter
In Fig. 11 another style of transmitter is shown. It is assembled on the front of a box. This front or cover swings on hinges, and can be opened so that the mechanism in the interior of the box may be gotten at easily.
A sectional view of this transmitter is shown in Fig. 12. A hole one inch and a half in diameter is cut in the cover (A). A round or square block (B) two inches and a quarter across and half an inch thick is made fast to the rear of the cover, and in this a hole is bored seven-eighths of an inch in diameter and one-quarter of an inch deep.
The sides and bottom of this hole are lined with flannel or felt, and a carbon button with roughened surface, as shown at F F, is made fast in it by a small machine screw and nut (F). A diaphragm (D) is cut from thin ferrotype plate, and a carbon button is made fast to the middle of it by a small machine screw or a rivet made from soft copper or brass. When the block (B) has been screwed fast to A, place some granules of carbon in the space (H); then lay the diaphragm over the opening, and make it fast with small screws or pins driven around the edge.
From a small tin funnel and a tin-can cap make a mouth-piece (C) by cutting a hole in the cap and slipping the funnel through it, then cutting the end of the funnel that projects through the hole and bending back the ears so that they lap on the inner side of the cap. These small ears may be soldered to the cap so as to hold the mouth-piece securely in place. From felt or flannel cut a washer the size of the can top and about three-eighths of an inch in width. Lay this over the diaphragm; then place the mouth-piece on it and fasten it to the door (A) with small screws. The use of this washer is to prevent any false vibrations in the mouth-piece affecting the sensitive diaphragm. Make a small hole through A and B and pass a bolt (E) through this hole, taking care to lap a thin piece of sheet-brass on the diaphragm (D), bending it over so that it will lie under the head of the bolt (E). The diaphragm must be scraped where the metal touches it, so as to make perfect electrical connection between D and E. At the rear end of E arrange a washer and nut (G), so that the current passing in at G travels through E and D, then through the carbon buttons and granules, and out at F.
From pine or white-wood one-quarter or three-eighths of an inch thick make a box four inches wide, six inches high, and two inches and a half deep. To the front of this attach a cover, which should measure a quarter of an inch larger all around than the width and height of the box. Use brass hinges for this work so that the cover may be opened. Fasten a transmitter to the front of the cover, or make one on the cover, as shown in Fig. 11, and attach the box to a back-board or wall-plate five inches wide and seven inches high made of pine or white-wood half an inch in thickness (see Fig. 13).
At the left side of the box cut a slot through the wood, so that a lever and hook may project and work up and down. The end of this lever is provided with a hook on which a receiver may be hung, as shown in Fig. 13, and the inside mechanism is arranged as shown at Fig. 14. A is an angle-piece of brass or copper, which acts as a bracket and which is screwed fast to the inside of the box. B is the lever and hook, which is cut from a strip of brass. The attached end is made wider, and an ear (C), to which a wire is soldered, projects down beyond the screw.
A view looking down on this lever and bracket is shown at Fig. 15. A is the bracket, B the lever, and E the screw or bolt holding the two parts together, with a thin copper washer between them to prevent friction. When the lever and bracket are made fast to the box, a spring (D) should be arranged, so that when the receiver is removed from the hook the lever will be drawn up to the top of the slot. A small contact-plate (F) is made of brass, and fastened at the lower end of the slot. On this the lever should rest when the receiver is on the hook. A contact-wire is soldered to this plate, which in turn is screwed fast to the inside of the box. This mechanism is part of a make-and-break switch to cut out and cut in the bells or telephone, and will be more clearly understood by referring to the diagram in Fig. 17. At the right side of the box a small push-button is made fast, and this, with two binding-posts at the top and four at the underside of the box, will complete the exterior equipment of one end of a line.
The construction of the push-button is shown in Fig. 16, A being the box and B the button which passes through a small hole made in the side of the box. C is a strip of spring-brass screwed fast to the box. It must be strong enough to press the small bone or hard rubber button towards the outside of the box. A wire is caught under one screw-head, and another one is passed under the screw-head which holds the other spring (D) to the box. When the button (B) is pushed in, it brings spring C into contact with D, and the circuit is closed. Directly the finger is removed from B the spring (C) pushes it out and breaks the circuit. This button is used only in connection with the call-bells, and has nothing to do with the telephone. The wires leading from the interior of the box pass through the wall-plate and along in grooves to the foot of the binding-posts, which are arranged below the box on the back-board, as shown in Fig. 13.
A buzzer or bell is made fast to the inside of the box, unless it is too large to fit conveniently, in which case it may be attached to the wall above or below the box.
The Wiring System
Fig. 17 shows the wiring system for this outfit, which, when properly set up and connected, should operate on a circuit or line several miles in length, provided that the batteries are strong enough.
This system may be installed in the box shown in Fig. 13, the flexible cord containing two wires being attached to the binding-posts at the top of the box and to the posts at the end of the receiver. This system differs from the one shown in Fig. 6 only in the addition of receivers T and T 2, and in the substitution of the automatic lever-switches (L S and L S 2) for the plain switches (S and S 2) in Fig. 6. When the line is “quiet” the receiver (R) should be hanging on the lever-switch (L S), which rests on the contact-plate (A). At the opposite side of the line the receiver (R 2) hangs on the lever-switch (L S 2), which in turn rests on the contact-plate (A A). This puts the bell circuit in service.
PLAN OF TELEPHONE CIRCUIT, COMPRISING RECEIVERS, TRANSMITTER, ELECTRIC BUZZERS OR BELLS, LEVER-SWITCHES, PUSH-BUTTONS AND BATTERIES FOR STATIONS NOT OVER FIVE MILES APART.
If the boy at the left wishes to call up the boy at the right he removes the receiver (R) from the hook (L S) and presses on the button (P B). This closes the circuit through the battery (C C C), and operates the electric buzzer or bell (E B 2) at the other end of the system, through line No. 1 and line No. 2. The operation may be clearly understood by following the lines in the drawing with a pointer. The boy at the left may keep on calling the boy at the right so long as the receiver (R 2) hangs on the lever (L S 2) and holds it down against the plate (A A). But directly the receiver (R 2) is removed, the lever (L S 2) flies up—being drawn upward by the spring (D) shown in Fig. 14—and closes the telephone circuit through the spring-contact (B B), at the same time cutting out the bell circuit. The boy at the left having already removed his receiver, the telephone circuit is then complete through lines Nos. 1 and 2 and batteries C C C and C 2 C 2 C 2, the boys at both ends speaking into the transmitters and hearing through the receivers. The contacts B and B B are made from spring-brass or copper, and are attached inside the boxes at the back, so that when the levers are up contact is made, but when down the circuit is broken or opened. In Fig. 18 an interior view of a box is shown, the door being thrown open and the receiver left hanging on the hook.
The arrangement of the several parts will be found convenient and easy of access. E B is the electric buzzer, L S the lever-switch, P B the push-button, T the transmitter, and R the receiver. Nos. 1, 2, 3, 4, 5, 6, 7, 8 are binding-posts or terminals, and B is the spring-contact against which the lever-switch (L S) strikes when drawn up by the spring (D).
The wires that pass from 6 to 7 and from 4 to 8 should be soldered fast to one side of the hinge, and those running from the terminals or nuts at the back of the transmitter (T) to 7 and 8 should be similarly secured. Small brass hinges are not liable to become corroded at the joints, but to insure against any such possibility the ends of several fine wires may be soldered to each leaf of the hinge, so that when the door is closed the wires will be compressed between the hinge-plates. For long-distance communication it will be necessary to install an induction-coil, so that the direct current furnished by the batteries, in series with the transmitter, can by induction be transformed into alternating current over the lines connecting the two sets of apparatus. This system is somewhat more complicated and requires more care in making the connections, but once in operation it will be found far superior to either of the systems hitherto described.
A Telephone Induction-coil
It will be necessary to make two induction-coils, as described in chapter iv., page 62, Fig. 8. A telephone coil for moderately long-distance circuits is made on a wooden spool turned from a piece of wood three inches and a half long and one inch square, as shown at Fig. 19. The core-sheath is turned down so that it is about one-sixteenth of an inch thick. This spool is given a coat or two of shellac, and two holes are made at each end, as shown in the drawing. The first winding or primary coil is made up of two layers of No. 20 double-insulated copper wire, one end projecting from a hole at one end of the spool, the other from a hole at the other end. This coil is given two or three thin coats of shellac to bind the strands of wire and thoroughly insulate them, and over the layer a piece of paper is to be wrapped and shellacked. The secondary coil is made up of twelve layers of No. 34 silk-insulated copper wire, and between each layer a sheet of paper should be wound so that it will make two complete wraps. Each paper separator should be given a coat of shellac or hot paraffine; then the turns of wire should be continued just as thread is wound upon a spool, smoothly, closely, and evenly, until the last wrap is on. Three or four wraps of paper should be fastened on the coil to protect it, and it may then be screwed fast inside a box. The core-hole within the coil should be packed with lengths of No. 24 soft Swedes iron wire three inches and a half long. In Fig. 19 the wires are shown projecting from the end of a spool, and Fig. 20 depicts a completed telephone induction-coil. The installation of the induction-coils is shown in Fig. 21.