TRAFFIC CONTROL TELEPHONE SYSTEMS Circuit - Sam Hallas

swing of the meter needle either side of zero during impulsing will be equal. This
shows that the positive impulses are of the same duration as the negative impulses.
To get the best result of this test it is preferable to use a heavily damped meter.
The relay SR is shunted by a rheostat and by adjusting this the release time of
SR can be varied thereby controlling the impulse ratio of the impulses transmitted. To
increase the time during which SR is on its back contact, turn the adjusting screw of
the rheostat in a clockwise direction. If, in the above test the swing of the meter needle
either side of zero is unequal, the rheostat should be adjusted until the ratio is correct.
It is assumed, of course, that the selector key originating the test impulses has itself
been checked for impulse ratio (prior to the test) as described in Part 2.
Circuit Breaker No. 4001-A
The Circuit Breaker opens the line battery circuit if excessive current is taken
from the battery, such as would be caused by a short circuit on the line or line circuit.
Its resistance is 2 ohms and it is normally adjusted to trip at 0.6 ampere. On
resetting the circuit breaker after it has been operated, care should be taken not to hold
it in forcibly by hand, so that, if the short circuit still exists, the circuit breaker will
have an opportunity to re-open the battery circuit.
Controller's Telephone Circuit
Since the controller is normally listening on the line, a head-set is the most
convenient form of receiver and transmitter to employ. The head-set is connected to
the talking circuit by means of plug and jack, so arranged that two sets can be used at
the same time. When no head-set is connected an alarm circuit is completed through
the jacks. This circuit may be used to operate an extension bell or an amplifier and
loud speaker. In the former case, way stations would require a magneto for calling the
controller while the second arrangement retains the normal method of operation, but
enables way station calls to be heard at a reasonable distance from the control position.
Foot Switch No. 4001-A
The Foot Switch is used instead of a speak-listen key, thus leaving both of the
Controller's hands free for recording messages. The Controller's talking circuit is of a
high efficiency type, so arranged that by operating the foot switch when talking, the
impedance matching is made optimum for both the transmitter and the receiver.
Page 5