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.