This is a moving iron voltmeter with a difference because it probably dates from the late Victorian period, say 1885?
I suspected, like many instruments, that the chap who'd calibrated the dial had left some evidence on the back of the dial. Unfortunately, although he'd pencilled in the fact, in very flowery script, that it was for 20 volts AC, he hadn't added his signature or the date.
At first I imagined that the voltmeter was a standard brass-cased type screwed to a wooden mounting plate, but further examination proved that this wasn't so.
Firstly the rear plate directly mounts the solenoid and the connecting bolts and secondly the plate isn't made of wood. It seems to be impregnated compressed paper, made from several hundred layers in fact. Maybe this material was better than wood when it came to insulating properties?
Much to my surprise, the meter worked, producing a full-scale deflection from 125mA of current. I used DC from a lab power supply to carry out the test and found that a reading of 20-volts coresponded to an applied voltage of about 18-volts DC. The RH picture shows the substantial operating coil wound, probably wound from double cotton insulated wire.
How is the meter different to "modern" types?
This meter relies on accurate zero reading by adjusting its mounting position. There isn't a hairspring and the pointer relies on gravity in order to indicate zero when there is no voltage applied across its terminals. This is pretty inconvenient as the meter must be fixed vertically and at precisely the correct angle otherwise the reading could easily be ten percent in error either way. Even my Victorian Cardews voltmeter has a mechanism to self-centre its needle.
The maker's name (or possibly that of the user?) is fairly crudely handwritten on a very ornate figured brass plate... N.C.S. The word "VOLTS" is also handwritten and the serial number "72601" implies a large manufacturing organisation. I need to do some research on this, or some advice, as I don't immediately know what this stood for.
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