46 
THE TELEGRAPH 
while yon listen by applying to your ear the instrument shown 
suspended to the desk ; at the same time notes may be taken 
with the hand that remains free. 
Before we follow the series of operations forming a complete 
call, let us examine the system of telephones used in the Broad¬ 
way office. This system belongs to the class of battery tele¬ 
phones, and thus allows these batteries to be utilized for the 
Fig. 17. 
calls to the subscribers by means of ordinary electric bells, such 
as represented on the desk of the office telephone, Fig. 16. 
The transmitter is Edison’s carbon telephone, which is based 
on the variations of electrical resistance produced by variations 
of pressure caused by the plate when the mouth-piece is spoken 
into. Thus, variations in the intensity of the current are pro¬ 
duced according to the inflexions of the voice, the vibratory 
movements of which are faithfully reproduced. The circuit is 
formed by the battery (two bichromate of potash cells), the 
transmitter, and the primary wire of a small Ruhmkorff coil 
