|P.O. ENGINEERING DEPT.
Issue 1, 4.11.58
ELECTRICAL CLOCKS - PULSE TYPE
Clock No. 70A - Description
Clock No. 70A was designed to transmit a pulse of approximately 30 seconds duration at every hour precisely and a derived pulse once every 24 hours.
The 24-hour pulse is controlled in duration by an external relay-set which is operated by the
1 hour pulse. The actual 24-hour pulse is transmitted via springs S3 and S4 which close a few minutes before 1 p.m. and open a few minutes after 1 p.m. The hourly pulses and the 24-hour pulse are transmitted on separate outlets from the clock. The clock was designed for use where routiner docket control equipment, subscriber trunk dialling meter-pulse generating equipment, or disputed accounts equipment is installed. Normally the clock is driven by
30 second pulses derived from the master clock (Clock No. 36) but it will operate to
`interrupted earth’ (075 second on, 0.75 second off) for resetting the clock (see par. 4).
30 second pulses derived from the master clock energize the electromagnet M attracting armature A to the left. Pawl D3 moves to the right and engages a tooth in the wheel W against spring tension. When the pulse ceases, the pawl is returned to its normal position by the restoring spring, carrying wheel W forward one tooth, or 1/120th of its circumference.
Fixed to the wheel W is an index disk Dl, carrying on its periphery a metal pin. During the revolution of the wheel and disk this pin short-circuits springs S 1 and S2 at every hour precisely and connects earth to the
1 hour pulse lead. The next 30 second pulse removes the short-circuit from these springs and the pulse on the
1 hour pulse lead ceases. To prevent false pulses being transmitted due to backlash in the clock gearing, springs S 1
and S2 are of different lengths. The wheel W drives, by means of a 24 to 1 reduction gear, the
24 hour disk D2 which is recessed on its periphery at a point marked 1 p.m. Two springs S3 and S4 are kept open, until one of the springs drops into the recess a few minutes before 1 p.m. The
closing of the springs prepares the clock to transmit the 24 hour pulse. Springs S3 and S4 open a few
minutes after 1 p.m.
3. The internal connexions of the clock are shown on Dgm. GMT 68. The resistance of the clock magnet is between
7.5 and 10 ohms, and the magnet is shunted to reduce inductive disturbances. Battery is fed to the clock via a resistor, mounted externally, to reduce the clock current to approximately 250 mA.
4. Advancing and retarding the clock
A key is fitted externally to the clock to provide the following
- To advance the clock automatically by connecting it to the interrupted earth pulse instead of the
30 second pulse when changing from G.M.T. to B.S.T.
- To retard the clock by disconnecting the 30 second pulse when reverting from B.S.T. to
The clock is only suitable for wall mounting.