Early Moving Iron Voltmeter

 Note the letters "N.C.S.", on the decorative plate; this is a trade mark for Nalder Brothers & Thompson.

It's fairly large, measuring about 8" or 20cm across.



 This is a moving iron voltmeter dating from 1902 or 1903.

I'd hoped 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 rear plate isn't made of wood. It seems to be a strip of impregnated compressed paper, wound round and round into several hundred layers. Maybe this material was better than wood when it came to insulating properties? Later meters use all-metal construction with ebonite, bakelite or tufnol posts and washers to insulate the input connections.

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.

This meter relies for 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 pointer is very light and has a wide flat construction to dampen its movement.

The maker's name 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've come across this early advert for Nalder Brothers & Thompson products from about 1903. And SNAP, my meter looks pretty well identical in design. The 35/- price of the voltmeter in terms of 2017 income is over £1000.


 Advertisement from 1903


 And after a little more investigation, here's the patent from 1902: Page 1/3


 Page 2/3


 Page 3/3


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