"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."
Fig. 169
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.
Fig. 170