CLOCK No. 36

B 1325
Issue 3, 13.2.59

Clock No. 36

1. Description
Clock No. 36 is an electrically controlled master clock having a 1 second beating pendulum. It provides 1 second, 6 second and half minute pulses for the operation of pulse clocks and telephone apparatus. The picture to the right shows the general appearance of Clocks No. 36 (Mark 234).

It is similar to the Clock No. 12 but is provided with an additional count wheel and contact springs for driving Veeder Clocks by impulses at 6 seconds.

It was introduced in 1925 due to the new automatic exchanges requiring a 6 second pulse for timing and metering purposes.

2. Operation
Half minute pulses of 200-500 ms duration are provided by a count wheel A, which has 30 teeth on its periphery. This wheel is rotated in a clockwise direction, one tooth for each complete swing of the pendulum, by means of pawl P attached to the pendulum rod B. On the count wheel A are two teeth set diametrically opposite to each other and cut much deeper than the remainder. They permit the pawl P to drop lower and engage the lever-rod D at every half-minute. The lever rod is attached to a contact spring C, which moves to the right under the action of the pendulum and pawl, closing contacts C every half-minute.

3. Six second pulses of 200-500 ms duration are produced by a count wheel Al, which has 30 teeth and is rotated in an anti-clockwise direction by means of pawl P1, in a manner similar to that of the count wheel A. Each third tooth is cut deeper than the remainder, allowing pawl P1 to engage with the lever rod Dl, closing contacts C1 every six seconds.

4. One second pulses of 200-500ms duration are produced by contacts F which are operated at each swing of the pendulum by the two axis E.

5. Clock drive
The swing of the pendulum is maintained by an electromagnet M having extended pole-pieces N, embracing the pendulum rod. Above the pole-pieces and attached to the pendulum rod is a soft-iron armature T. Whilst the arc of oscillation of the pendulum lies outside a pre-determined minimum, the toggle it trails over the notched agate S attached to the pendulum rod; but when the arc has declined to the minimum, the toggle it engages in the notch in the agate and, on the return swing of the pendulum, the toggle is raised and it's associated contact C2 closes. The driving battery is connected momentarily to the electromagnet M, and an impulse is imparted to the pendulum. A potential of 4 volts connected directly across the coil is required for satisfactory operation (see Diagrams GMT 32 and in addition, GMT 39/0 for Mk. 4, GMT 39/1 for Mk. 5, and GMT 39/2 for Mk. 6).

Fig 2. Mk 4 Mechanism Fig 3. Mk 5 & 6 Mechanism

6. Mounting
The clock must be mounted in a vertical position. To ensure this, a plumb register is fixed to the base inside the clock, so that when the clock is vertical the point V of the pendulum is directly over the plumb register.

7. Adjustment
The swing of the pendulum is regulated by raising or lowering the brass-encased lead bob by means of the rating nut W. The clock will gain if the bob is raised, or lose if it is lowered. It is possible to adjust the Clock No, 36 to keep G.M.T. to an accuracy of 8 seconds variation per week.

8. Sparking at contacts
A special resistor, having a resistance of 2,000 ohms, is fitted and wired to each set of pulse contacts to reduce sparking (see Diagram GMT 32).

9. Clock No. 36, Mk. 4 (Introduced 1938)
This design of the clock is similar in operation to previous models but differs in construction in the following respects:-
(a) Terminals and wiring
(b) Spring-sets
(c) Spark-quench.

The terminals are located at the top of the clock immediately above the casting and the clock is wired to Diagram GMT 39/0.
The spring-sets are simple 'make' type instead of 'make before break', which enables a standard spark quench to be used.

10. Clock No. 36, Mk. 5 (Introduced 1955)
This design is similar to the Mk. 4 version but differs in that synchronizing equipment (items K, L, J, H and G, of Fig. 2), which as no longer used, is not provided. The clock is wired to Diagram GMT 39/1.

11. Clock No. 36, Mk. 6 (Introduced 1956)
This design is similar to the Mk. 5, but differs in that the Wiring is now in three single colours wired to Diagram GMT 39/2. A label diagram LD 152 is also pasted inside the clock, showing the wiring. To avoid damage to the clock during transport, special mild-steel details have been fitted to bold the pendulum instead of the wooden blocks used previously, and the mechanism casting is now bolted to the clock case.

Drawing No. 36 & 39.


Clock No. 36 Mk 4 Feet & Inches Metric
Height 4'  7" 140cm
Width 1'  2.5" 37cm
Depth 7" 18cm

The pendulum suspension springs are made of 8 thou (0.2mm) spring steel.  The springs are sized 1'" x '" with ⅛" holes, '" apart, centered.  The holes must be made with a punch and a horologists staking set would be the right tool to use.  This is to ensure the hole is clean and undistorted.  Warning - Spring steel is also very difficult to drill!

How to make a home made staking tool - fix them broken suspension springs.

Specification (Mark 6) - PR479 (was S28).


Diagrams and Drawings
  • Clock No. 36 - GMT32

  • Clock No. 36 Mark 4 - GMT39/0

  • Clock No. 36 Mark 5 - GMT39/1

  • Clock No. 36 Mark 6 - GMT39/2

  • Clock No. 36 Time Distribution using GMT34 - GMT34/0 and GMT34/1

  • Clock No. 36 Time Distribution using GMT35 - GMT35/0 and GMT35/1

  • Clock No. 36 Time Distribution of GMT 1925 - GMT26

  • Clock No. 36 Time Distribution of GMT 1927 - Extract for TI

  • Clock No. 36 30 second pulse circuit for Clock Switching Unit - GMT64

  • Clock No. 36 Mark 5 and Mark 6 mechanism - P/T 39/0 and P/T 39/1

  • Clock No. 36 Mark 4 case - 7587/0
    Clock No. 36 Mark 5 case - 7587/1

  • Clock No. 36 Mark 6 case details - P/T 29 (was 90817) and 7987/2

  • Clock No. 36 Case - P/T 36


Clock No. 36 Mk 5 with door closed and pendulum in shipping position Clock No. 36 Mk 5 with door open and pendulum in working position


Clock No. 36 Mk 4

Cabinet showing pendulum installed
Cabinet showing pendulum is transport position
Closeup of manufactures plate
Closeup of main springs with pendulum installed
Closeup of main contact sets
Closeup of synchroniser unit
Central plate supporting all the contact sets
Bottom of cabinet showing plumb register and low fixing thumbscrew (this could be slackened to allow side to side movement of the cabinet
Connection strip at top of cabinet


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Last revised: January 07, 2024