MAGNETA CLOCKS

Towards the end of the 19th century the Post Office decided that centralised time would be beneficial in their large telephone exchanges and sorting offices.  They decided to buy the Swiss Magneta systems which used weight driven master clocks.

These master clocks gave a one minute impulse, whereas today the impulse clocks all work on the 30 second principle.

Engineering Instructions
MISCELLANEOUS
TIME
B 1381

ELECTRICAL CLOCKS-IMPULSE TYPE
Magneta System

1. General
There are still in existence several installations of the old Magneta. 1-minute impulse system, which employs a master impulse generator and is described in T.I. XI, paragraphs 19-20, a copy of which should be retained where necessary.  Although these installations continue to give satisfactory service, the system is obsolete.

2. Loading of system
The majority of Magneta master clocks are capable of operating 100 units with possible overload of 20 per cent.  Table 1 shows equivalent units for clocks of different diameter.

TABLE 1
 

Owing to the age of the master clocks, however, over loading should not be resorted to if it can be avoided.  The 1-unit clock, which has a resistance of approximately 30ohms, requires a voltage of 0·4V. for satisfactory operation.

3. Replacement by standard system
The replacement of the Magneta system by the standard half-minute impulse system using a "Clock No. 36" should be considered when any of the following conditions arise:-

1. when the system commences to function unsatisfactorily, due to general deterioration as distinct from overloading.

2. when additional secondary dials are required and the impulse generator is already fully loaded.

Condition (b) may sometimes be met by reducing the number of 1-minute secondary dials and providing standard 1-minute secondary dials with a "Clock No. 36" to make up the number required.  The recovered 1-minute dials should be packed carefully and returned to the stores department for use on other installations.

References:- T.I. XI, paragraphs 19-20.
 

Magneta Master Clock

The Magneta master clock combines the functions of a generator of electric impulses and of a timepiece.

The time-train is weight-driven, and is regulated in a similar manner to the average pendulum clock. Incorporated with and driven by the time-train is a trip gear which is actuated mechanically once a minute.

At the end of every minute this trip gear gives a sudden thrust or pull to the horizontal rod R; the left hand extremity of this rod communicates the movement to the armature A of the powerful compound permanent magnet M. The rapid right and left motions of this armature in the magnetic field of the compound permanent magnet generate in the armature winding alternate current impulses which pass out to the coils of the secondary dials connected to the master clock.

In the event of the clock running down, the weight presses on one end of the rocker C in the base of the clock and causes the other end of C to rise, thus arresting the movement of the pendulum.

The weight, on being wound up, actuates at the top of its travel the rod A, which in turn depresses the rod B. The depression of B raises the plate P into a position such that it engages with a "Snail" 8 on the winding arbor and arrests the motion of the winding handle.

MAGNETA SECONDARY DIALS
The electrical portion of the dial consists of an electromagnet M, the cores of which are polarized by the permanent magnet P.

Alternate positive and negative currents passing through the coils of the electromagnet cause the armature A, which is pivoted at its centre, to be depressed on the right or left according to the direction of the current through the coils.

Attached to the armature (Figure 1) is a vertical arm, the upper end of which moves between two pins fixed to the piece carrying the propelling pallets B. These in turn, acting on the toothed wheel fixed to the canon pinion arbor C, cause the latter to move forward 6° (-„¹„th of its circumference) for each impulse received.

In Figure 2, the vertical arm terminates in a ring, on the upper portion of which is a projecting pin. This pin can move inside a loop on one end of the spiral spring, the other end of which is attached to the piece carrying the pallets B. These pallets transmit the motion to the canon pinion C in a manner similar to that described in connexion with Figure 1.

The usual 12-1 gearing for the hour hand is also shown in Figures 1 and 2.

Figure 1
Magneta Secondary Dial No. 1 mechanism

Figure 2
Magneta Secondary Dial No. 2 mechanism
 

Last revised: January 07, 2024