Cambridge Instruments Thermionic Voltmeter [Moullin valve voltmeter pattern D]. Housed in polished wooden case with ebonite front panel. This instrument has 2 ranges, 0-4/20 Volts. Working. Cambridge Instrument Company Ltd, UK, date about 1922/3 [the patent was granted in 1923]
This is one of many instruments collected by Jack Davidson C Eng FIEE which I have been given by Dr Graham Winbolt
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 a pattern A voltmeter for AC and DC use, a pattern B, a pattern C (double range), a pattern D (AC), and a pattern P(with a probe).