b. Application of operating
practical magnetic-diaphragm receiver.
(1) In the early telephone receivers devised by
Bell, a bar magnet was used to supply the permanent
magnetic field (A, fig. 18). Bell actually used two
receivers of this type in his early telephone system, one
serving as the transmitter and the other as the receiver.
Later, the efficiency of the receiver was improved
greatly by using a horseshoe magnet in place of the bar
magnet, as shown in B. Because the length of the
magnetic path is much shorter in the horseshoe magnet,
the magnetic field is concentrated in the region between
the poles. This increases the pull on the diaphragm for a
given value of current, and therefore produces sound
waves of greater intensity. The modern receiver unit
incorporates a modification of the horseshoe magnet.
This, with the use of better magnetic alloys, has
improved the design and performance of the receiver.
(2) In a later chapter it will be shown that the receiver winding occasionally must have direct current flowing through
it. Because of this requirement, the receiver is connected in such a manner that the field produced by the direct current in the
coil aids the field of the permanent magnet. This increases the strength of the field, and results in a stronger pull on the