Power source

In the first experimental telephones the electric current that powered the telephone circuit was generated at the transmitter, by means of an electromagnet activated by the speaker’s voice. Such a system could not generate enough voltage to produce audible speech in distant receivers, so every transmitter since Bell’s patented design has operated on a direct current supplied by an independent power source. The first sources were batteries located in the telephone instruments themselves, but since the 1890s current has been generated at the local switching office. The current is supplied through a two-wire circuit called the local loop. The standard voltage is 48 volts.