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.
Fig. 171
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.
