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Here’s a gadget to get your teeth into, or
rather, get your (false) teeth out of, had you been around in the fifties and sixties.
At least that’s my best and only guess about what the Nattex Copper Forming
Dental Outfit was designed to do. So let’s begin with some certainties. It was
made in England by Cottrell & Company, located in London W1. The date of
manufacture is a little harder to gauge but the components inside the box
strongly suggest that it was made somewhere between the mid fifties and the
early sixties. Now for the big one, what on earth is it for?
I welcome clarification from anyone who
actually knows, or has used one of these things but at this point all I can say
for sure is that it is a variable mains-powered low voltage DC power supply,
presumably used for electroplating copper, in some process involved in the
manufacture of dentures. The power supply part isn’t in doubt but the only
evidence I have for the latter assumption, that it has something to do with
dentistry, is in the engraved label on the front panel. That and the three
terminals on the front, which look like they might be for connecting plating
bath electrodes; the meter, which shows current flow, and the variable control
that probably adjusts the speed of the process. 
The front panel controls include a
three-position range switch for the meter, labelled x10, x5 and x1. There’s an
on/off switch and indicator lamp for the mains supply and a knob connected to a
multi-turn variable resistor or rheostat that controls current flow. Inside the
case there are only three main components, a mains transformer, a finned Selenium
rectifier, and the heavy-duty rheostat. The Selenium rectifier was the key to
dating the device. They were commonplace in mains-powered electrical equipment
up until the early 1960s when they were replaced by silicon diodes; this was no
loss, believe me! Selenium rectifiers were evil things with a limited life, and
when they failed -- which was all the time -- they emitted a foul and
long-lasting smell. To be fair it did make finding some types of fault a lot
easier... The rectifier’s job is to convert the low voltage AC coming from the
transformer’s secondary winding into a DC voltage. After passing through the
rheostat, the voltage ends up on the output terminals on the front. You can’t
see it from the photos but that rheostat is quite a thing. Basically it’s an
insulated tube, covered in a winding of resistance wire. A screw-driven
contact, connected to the knob on the front panel, travels up and down the
tube, varying the resistance between the moving contact and two ends of the
winding as it goes.