TRAFFIC CONTROL TELEPHONE SYSTEMS Circuit - Sam Hallas

Page 24
General Description
PART 3
4301-A SELECTOR
4304-A SELECTOR
4303-A Selector
Fig. 9.
The function of the selector is to provide a quick and reliable means to calling
selectively one of a large number of way stations on the same telephone line circuit
without producing a signal at the other stations.
The coils of the selector are wound to 21,000 ohms and are tuned to 32 cycles
per second by a 1.25 μF capacitor. The impedance of the selector and capacitor at its
operating frequency of 3½ cycles is approximately 35,000 ohms and the impedance
at 800 c/s (speech frequency) is over 1 megohm. The loss to speech due to the
selector is therefore negligible and invariably is considerably less than the loss due to
line leakage. The selector with its associated capacitors and bell (or if desired, a
combined set which includes the telephone circuit) is contained in a wooden box
known as a selector set.
The selector (Fig. 9), shown with the glass cover removed, is a polarised relay
arranged to advance a ratchet wheel, tooth by tooth, as successive impulses of
opposite polarity are received. The ratchet wheel carries a code wheel into which
code pins can be fitted in positions corresponding to any ratchet wheel tooth.
The code wheel carries a contact arm which is in contact with the ringing
terminal in one position of the code wheel and with the time terminal in another
position.
The code wheel is set so that the same total number of steps is necessary to
advance the code wheel of all selectors on the same line to the ringing position.
Similarly a series of 22 uninterrupted impulses will advance all selectors on the line to