Fig. 324. DIAGRAM OF BELL TELEPHONES AND LINE WITH EARTH CONNECTIONS.
Fig. 325. SECTION OF BELL TELEPHONE.
The above gives the essential features of the Bell telephone. In
practice the telephone is used only as the receiver. As transmitter a
microphone is employed. To give the current a battery, generally of the
open circuit type, is used, and the current in the line is an induced or
secondary one.
The microphone which is talked to, and which is the seat of the current
variations which reproduce original sound, is termed the transmitter,
the telephone in which the sounds are produced at the distant end of the
line is termed the receiver.
Fig. 325 shows the construction of the Bell telephone in universal use
in this country as the receiver. M is a bar magnet, in a case L L. B B
is a bobbin or coil of insulated wire surrounding one end of the magnet.
D is the diaphragm of soft iron plate (ferrotype metal), and E is the
mouthpiece. The terminals of the coil B B connect with the binding
screws C C. The wire in the coil is No. 36, A. W. G., and is wound to a
resistance of about 80 ohms.