Cambridge Instruments Thermionic Voltmeter
Moullin valve voltmeter type A

Serial No.L-230838

Cambridge Instruments Thermionic Voltmeter [Moullin valve voltmeter type A]. Housed in polished wooden case with ebonite front panel. Scaled 0 to 5 Volts and -1 to +5 dBs. Meeter pointer has corroded and fallen away. Working. Cambridge Instrument Company Ltd, UK, date about 1930? The patent numbers on the front of the instrument date from 1923.

This is one of many instruments collected by Jack Davidson C Eng FIEE which I have been given by Dr Graham Winbolt.

As received the aluminium pointer had corroded and disintegrated but the movement was still OK. I have managed rather inexpertly to attach another and establish that the meter functions.

Instructions on outside of wooden case.

Interior view.


Eric Balliol Moullin, [1893-1963] was a Cambridge Professor of Electrical Engineering and President of the Institution of Electrical Engineers [IEE]. He invented the first vacuum-tube voltmeter which he described in a paper to the IEE in December 1922. It was manufactured and marketed by the Cambridge Scientific Instrument Company. Several versions were produced, varying from an pattern A voltmeter for AC and DC use, to pattern B, a pattern C (double range), a pattern D (AC),and a pattern P(with a probe).

Simplified diagram

The design is remarkably simple, requiring only a 6 Volt supply which is adjusted to provide a negative bias on the grid and a predetermined voltage across the filament.