Complete Installation.—To install a more complete system requires, at each end, a switch, a battery and an electro-magneto bell. You may use, for this purpose, a bell, made as shown in the chapter on bells.
Fig. 84 shows such a circuit. We now dispense with one of the line wires, because it has been found that the ground between the two stations serves as a conductor, so that only one line wire (A) is necessary to connect directly with the telephones[p. 117] of the two stations. The telephones (B, B', respectively) have wires (C, C') running to the pivots of double-throw switches (D, D'), one terminal of the switches having wires (E, E'), which go to electric bells (F, F'), and from the bells are other wires (G, G'), which go to the ground. The ground wires also have wires (H, H'), which go to the other terminals of the switch (D, D'). The double-throw switch (D, D'), in the two stations, is thrown over so the current, if any should pass through, will go through the bell to the ground, through the wires (E, G or E', G').
Now, supposing the switch (D'), in station 2, should be thrown over so it contacts with the wire (H'). It is obvious that the current will then flow from the battery (I') through wires (H', C') and line (A) to station 1; then through wire C, switch D, wire E to the bell F, to the ground through wire G. From wire G the current returns through the ground to station 2,[p. 118] where it flows up wire G' to the battery, thereby completing the circuit.
The operator at station 2, having given the signal, again throws his switch (D') back to the position shown in Fig. [84], and the operator at station 1 throws on his switch (D), so as to ring the bell in station 2, thereby answering the signal, which means that both switches are again to be thrown over so they contact with the battery wires (H and H'), respectively. When both are thus thrown over, the bells (G, G') are cut out of the circuit, and the batteries are both thrown in, so that the telephones are now ready for talking purposes.
Microphone.—Originally this form of telephone system was generally employed, but it was found that for long distances a more sensitive instrument was necessary.
Light Contact Points.—In 1877 Professor Hughes discovered, accidentally, that a light contact point in an electric circuit augmented the sound in a telephone circuit. If, for instance, a[p. 119] light pin, or a nail (A, Fig. [85]) should be used to connect the severed ends of a wire (B), the sounds in the telephone not only would be louder, but they would be more distinct, and the first instrument made practically, to demonstrate this, is shown in Fig. [86].