|P.O. ENGINEERING DEPT.
Issue 2, 27.4.49
ELECTRICAL CLOCKS - IMPULSE TYPE
Clock No. 44
44 (chargeable time clock) is the standard timing device for auto-manual, C.B. and C.B.S. exchanges, though at certain centres chargeable time indicators are fitted. This Instruction gives a general description of the clock and its facilities. A more detailed description of the mechanism and adjustments is given in TELEPHONES, Automatic, B 5150 (M.A.I. 50) and the full facilities provided by timing cord circuits incorporating
Clock No. 44 are included in TELEPHONES, General, M 1901.
The clock, which is illustrated to the right, is of the Veeder type and is fitted, together with a combined start and re-set key and a standard lever-type speak key, on a key-mounting plate. Each of the three assemblies, i.e., clock mechanism, start and re-set key and speak key, may each be removed from the key-mounting without disturbance of the other assemblies.
3. Clock mechanism
The mechanism is operated on the reverse-drive principle by an electromagnet, actuated by a 6-second pulse from a master clock. Two drums or number wheels engraved
0-9, one wheel recording tenths of a minute and the other recording minutes, provide for a total reading of nine minutes. Mounted directly over the number wheels is a small window through which the elapsed time of the call may be read. The two number wheels are fitted with cams which operate spring-sets to provide the facilities specified in par. 8.
4. Combined start and re-set key
The combined START and re-set key comprises a rotary start key (3-position at sleeve control exchanges and 2-position at other exchanges), and a vertical plunger type re-set key.
5. An annular collar surrounding the stem of the re-set key is capable of being rotated from the central STOP position, to the START position on the left, thereby operating a number of spring-sets. The 3-position key used at sleeve control exchanges has a second start position by turning the collar to the right for timing coin box calls.
6. The plunger of the re-set key can be depressed only when the rotary key is in the central or STOP position ; this operation lifts certain springs out of engagement with cams and resets the number wheels to their normal position.
At sleeve control exchanges all clock mounting plates are engraved 0RD : STOP :
CB, the last being in red. At other exchanges the first clock mounting plate in each key-shelf is engraved START : STOP, the subsequent plates being
The Clock No. 44 indicates the duration of a call in minutes and 1/10 minutes. The cam operated spring-sets provide the following facilities:-
- Operation of a relay which applies a three-pip tone to the line approximately 12 seconds before the end of each 3-minute period, i.e., when the clock indicates
2.8, 5.8 and 8.8 minutes.
- On calls from ordinary subscribers, a time check lamp glows to indicate the approaching end of a 3-, 6-, or 9-minute period, the lamp glowing at
2.8 to 3.0 minutes, 5.8 to 6 minutes and 8.8 to 9 minutes. At the end of 9 minutes, the
drive magnet circuit is disconnected and a flicker signal is given on the lamp. Further periods of timing in excess of 9 minutes are obtained by resetting the mechanism.
- On calls from coin-box users, the time check lamp starts to flicker at 28 minutes. The clock continues to step (provided the caller remains in circuit) and the lamp to
flicker until the start key is restored to the STOP position, which action is usually effected when the clock indicates
3.0 (or as soon after as possible) and the call is terminated. If, however, an additional period of time
is paid for, the clock is reset and then re-started for timing the further period, the flicker signal being re-connected when the clock again indicates
9. The normal position of the number wheels is 9.9, and the first impulse brings the wheels to
0.0. This is necessary in order to ensure that the three-pip tone is transmitted not earlier than 12 seconds before the expiry of a 3-minute period, should a 6-second pulse be encountered immediately the START key is operated.