Part 24

Producing a tone is merely a matter of making something vibrate with the required frequency. It may be a piano string, or a tuning fork, or a reed of an electric buzzer, or the diaphragm of a telephone receiver. If it vibrates 256 times a second, it will produce the same tone as middle C on a piano; if it vibrates 512 times a second it will produce the C which is an octave above, and if 128 times a second an octave below middle C. The human voice is produced by vocal cords in the throat, which vibrate with the proper frequency to give any required tone. But how can we make the human voice act as an interrupter of the primary circuit? An examination of the telephone transmitter will supply the answer to this question.

The boys after taking the transmitter (Fig. 165) apart proceeded to make one which should answer the purpose as follows: A block of wood about one inch thick and three inches square (Fig. 166), A, was hollowed out, making a cone-shaped cavity about one half inch deep and one inch broad. This cavity was filled with small pieces of graphite, G, made by cutting up a lead pencil. An old tin-type, D, was laid over this as a diaphragm and tacked around the edges. A binding post, E, passed through the block, its head being buried in the graphite at the bottom of the cavity. The binding post F furnished contact with the tin-type. One dry cell was placed at B and the sensitive ammeter was connected at C. The needle showed that although a small current was passing it was constantly varying in strength. Tapping upon the table, walking across the floor of the room, shouting, and particularly whistling, caused variations in the conducting power of the graphite and consequently variations in the current strength. This is precisely the condition we wished to produce in the primary circuit.

We next substitute for the ammeter at C the primary and secondary coil of the telephone. In Fig. 167 T is the transmitter, B is a battery of two dry cells, P is the primary winding of the coils, and S is the secondary winding. To this a telephone receiver, R is connected by wires long enough to reach into another room. A person holding the receiver at his ear could hear everything said or done in the room where the transmitter was almost as plainly as though he were present in the room.

Two such transmitters were made and the second one was placed in the room where the receiver had been, while a second receiver was installed near the first transmitter. The arrangement is shown in Fig. 168. T is the transmitter at one end of the line and T' the transmitter at the other end. B and B' are the batteries at each end, P and P' the primary coils, S and S' the secondary coils and R and R' the receivers. With this arrangement two persons carried on a conversation with perfect ease, holding the receivers to their ears, presenting their mouths to the transmitters and speaking in moderate tones. H and H' are hooks upon which the receivers are to be hung when not in use. These hooks act as switches to open and close the primary circuit. A spring normally pushes the hook upward and closes the circuit, but while the receiver is hanging upon it the circuit is open at this point. Thus the battery is saved from running down when the telephone is not in use.

The wires were finally extended from the mill to the cottage and this equipment was installed at each end.

It will be noticed that the secondary circuit includes two receivers and two secondary coils besides the wire of the lines to offer resistance.

The receivers offer 75 ohms of resistance each. The secondary coils offer 250 ohms each and the line wires between the mill and the cottage offer 100 ohms. This makes a total of 750 ohms for the secondary circuit. But the rapid alternations which are induced in the secondary circuit impede the electric current ten times as much as the resistance already mentioned.

When considering alternating currents passing through coils of wire we are obliged to take into account two kinds of resistance:

1. Ohmic resistance.

2. Impedance.

"You boys understand the resistance to the flow of the electric current, which we have so often measured in ohms. But I want to show you that there is another kind of resistance which alternating current meets. Here is a coil containing 1000 feet of No. 20 copper wire. I throw on to it, for only an instant, the 110-volt direct current, and the ammeter reads 11 amperes, showing that it offers a resistance of 10 ohms to the direct current. I now throw on the alternating current, and the ammeter shows only a small fraction of an ampere. The surging of the current back and forth induces a counter electro-motive force, in the successive layers of the coil, which we call impedance. In the experiment which we have just performed impedance is fifty times as important a factor as ohmic resistance. Impedance depends chiefly upon the frequency of alternation. The impedance in telephone circuits is particularly large because of the extremely high frequency of the alternations produced by the tones of the human voice, these being usually not far from ten times as rapid as those of alternating currents in common use.

"We may estimate the total resistance of our telephone circuit as equivalent to 7500 ohms.

"Our secondary coils have forty times as many turns as the primary coils, and by means of them the voltage is stepped up to somewhere near one hundred on open circuit. When closed through the line, however, the voltage drops down to about ten. The result is that the actual current which passes between the cottage and the mill when we telephone is not far from .001 ampere. We may, however, hear a whisper transmitted by .000001 ampere or less.

"The tone E´ which is produced by the tenth key above middle C on the piano, is the one most readily heard over the telephone. It is produced by anything which vibrates 640 times per second."

We used No. 12 galvanized iron wire for our telephone lines. Two miles of No. 12 copper wire would offer 16 ohms of resistance. The iron wire offers about 100 ohms. But this is a trifle when compared with the total resistance. We used a double metallic circuit so as to avoid the effects of inductance from our electric lighting circuit.

The next thing that we were obliged to consider was some arrangement for calling persons to the telephone for conversation. We decided to use magnetos and alternating current bells. Fig. 169 shows the essential mechanism of the bells. The bell at each end of the line consists of two gongs a, b and a´ b´, with a hammer c, c´ between them. This hammer is attached to an iron armature h, h´, pivoted over the electro-magnets, m, m´, in such a way that it rocks back and forth when an alternating current passes through the lines d e, f g. The bells at both ends of the line always ring together, since they are connected in series. A magneto (Fig. 170) is situated at each end of the line. This, as has been previously explained, is a generator of electricity, in which the field is furnished by steel magnet, M. The armature A is a coil of wire whose ends are in contact with the leading out wires d and c by means of brushes which slide upon rings. The armature is revolved by hand. The crank and cog wheels employed to produce high speed are not shown in the figure. By turning the armature rapidly this magneto will develop 60 volts e. m. f. on open circuit. The magnets of the bells are wound with a very large number of turns of very fine wire, so that .025 ampere is sufficient to ring them.

Figure 171 shows how the magneto at either end of the line is introduced into the circuit for the purpose of ringing the bells. B and B' represent the bells, m and m' the magnetos, and P and P' represent switches. Springs push them upward so that they normally close the circuit through the bells. When a person at P wishes to call another at P' he pushes the switch P down so as to bring his magneto m into series with the bells. When now he turns the crank and generates the electric current, both bells ring. His own bell serves the purpose of telling him that the line is operating all right. The other bell calls the party desired for conversation. As soon as the operator removes his finger from the switch P the spring throws it upward again, leaving his bell in circuit, so that he may be called at any time, but cutting out of the circuit his magneto, which would introduce unnecessary resistance.

The same wires which carried the current for ringing the telephone bells carried also the current for operating the telephone receiver. When the receiver is removed from the hook it releases a twofold switch. This serves the double purpose of closing the primary circuit through the local battery and substituting the telephone receiver circuit for the bell-ringing circuit upon the line.

We used fifty chestnut poles to carry our line between the mill and the cottage. Each pole had a cross bar, on one end of which the electric light and power wires were carried and on the other end the telephone wires. Glass insulators prevented the wires from coming in contact with the wood of the cross bars. The necessity for this was impressed upon the boys by something which happened while they were stringing the wires. The telephone apparatus at the mill had been installed and the two leading out wires had been connected to it. One of these was coiled up on the floor, while the other had been strung along upon the poles for half a mile, but had not yet been attached to the insulators on the poles. While the boys were lunching at the mill, one of them gave the crank of the magneto a turn, when, to the astonishment of all, the bell rang. The circuit had been completed through the damp wood of the mill, through the damp wood of some of the poles, and through the earth. After lunch the wire, so far as it had been strung, was fastened to the insulators upon the poles. But when some one turned the crank of the magneto the bell still rang. We walked along the line to see where the difficulty was. We found the end of the line about half a mile from the mill dangling free from the ground, but touching a tall spear of grass. When this was moved away from the spear of grass the magneto could no longer ring the bell. The slight current required to ring this bell--.025 ampere--had found its way through the spear of grass, through the woodwork of the mill and through the earth.

We had no sooner got the two telephone wires properly strung and attached to the hundred glass insulators when a thunder storm came up, and drove us back to the mill for shelter. Pretty soon the bell rang and we, supposing that some one at the cottage was trying to call, went to the instrument, but could get no response, nor could we make the bell ring. Lightning had sent an alternating current over the line which rang the bell, but the strength of the current was too great for our coils of fine wire and one of them was burned out, as we say. In other words, the wire had melted at the point where it offered the greatest resistance.

The burned-out coil was replaced, and then we installed lightning arresters which were of two kinds. The first were simply fuses which were introduced into the line to protect it against any current too large for the apparatus to carry, and the second was a plate, c (Fig. 172). These are to be found upon the top of the magneto cases. A wire is connected with c, and its other end is grounded by being connected with a piece of iron pipe which is driven deep into moist earth.

The plate a b is inserted in the line, and the gap between this and the plate c offers sufficient resistance so that the telephone circuit suffers no leakage at this point, but lightning has such extremely high tension that it readily passes across this gap and finds its way to the earth without damaging the instruments.

We have already noticed that our alternating current dynamo, which produces 60 vibrations per second in the telephone receiver, causes it to give a tone very nearly like the C, which is two octaves below middle C upon the piano. C requires 64 vibrations per second. We may speed up our dynamo so as to make it yield a tone exactly like C or even above it.

Dr. Cahill of Holyoke, Mass., has devised an organ in which alternating current dynamos produce the necessary number of vibrations for each tone. The name telharmonium has been proposed for this organ. It has a separate dynamo for each tone, each dynamo having a frequency corresponding to the tone required of it. The dynamo, for instance, which produces middle C makes the electric currents surge back and forth 256 times a second, and this causes the diaphragm of a telephone receiver to vibrate 256 times a second, and this sends forth 256 air waves per second, and when these reach our ears we recognize the tone we call middle C. The frequency of alternation in a dynamo may be increased by either increasing its speed of revolution or by increasing the number of coils upon its armature.

Mr. Cahill's great organ looks like a large machine shop with many counter shafts geared so as to run at different speeds. On each shaft are a large number of little dynamos whose armatures have various numbers of coils. The organist, who may be far removed from this "machine shop," fingers an ordinary keyboard. Each key opens and closes a switch, thus bringing into action its own dynamo.

If the key which is known as C, one octave below middle C, is pressed down, a switch closes the circuit between the telephone and a dynamo which gives 128 double alternations of current.

The tone which is produced by 128 vibrations per second is the one most often heard from a man's voice in ordinary conversation.

Another key brings into action upon the same telephone receiver--and at the same time if desired--a dynamo which gives twice as many alternations per second and produces the tone most often heard in female conversation. It is middle C.