WW1 800 Cycles Testing Set Acoustic Oscillator No.10 Telephone Tester

This amazing looking piece of vintage military scientific instrument is a WW1 800 Cycles Testing Set Acoustic Oscillator No.10 Telephone Tester, and it is a real enigma. We believe it was probably made by Marconi but there are no manufacturer's label or marks to confirm this.

An expert advised us that this instrument dates to WW1 and would have been used for testing trench telephone lines but we have no provenance for this. However, it is fitted with a 1920s PM2 valve, which obviously postdates WW1. It may well be that the standard British B4 valve base had a bright emitter valve fitted which was replaced with the PM2 in subsequent years.

This interesting test kit contains a brass vibrator, which transforms DC power into AC waveform.

It measures in Periods/sec, which is an arcane unit for cycles/sec or Hertz.

It has a label which states:-

This instrument produces an approximately pure note of frequency, about 800 Periods per Sec, in telephones connected directly or indirectly to the terminals marked 'Signals.' 

An Audio Oscillator generates tunable waveforms and are used, for example, in the generation of test signals for test benches, and the generation of control signals for audio effects. 

A lovely piece of vintage telegraphy technology!