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
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