"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." [Illustration: 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. [Illustration: Fig. 170] 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. [Illustration: Fig. 171]