Master/Slave Electric Clock Systems

Movement & mechanism

• Master clock: a precision pendulum regulator fitted with electrical contacts that close at set intervals (commonly every half-minute or minute)
• Slave dials: simple electromagnetic mechanisms with no independent timekeeping ability, advancing only on receipt of an impulse from the master
• Wired distribution network connecting one master to many slave dials across a building or site

Identification

• Slave dials typically have a very simple movement with no mainspring, weight or independent escapement — only an electromagnetic stepping mechanism
• Master clocks are generally substantial precision regulators with visible electrical contact mechanisms
• Wiring terminals on the case back of a dial clock, rather than a winding arbor, indicate a slave dial

Terminology

• Master clock — the precision timekeeping source in the system
• Slave dial — a synchronised display unit with no independent timekeeping capability
• Impulse — the periodic electrical signal advancing a slave dial

Known variations

• Half-minute impulse systems
• Minute impulse systems
• Synchronome-type free-pendulum master clocks

Collecting notes

Master clocks (particularly those by established scientific-instrument makers) are collected as precision regulators in their own right; slave dials are collected chiefly for institutional provenance (railway, factory, public building) rather than mechanical sophistication.

Sources & references

1. General horological literature on electrical timekeeping distribution systems; individual manufacturer histories require further research.

Where sources disagree, the disagreement is stated rather than resolved silently.