Electrical Engineering at UNSW really provides some really interesting salvages from time to time – today, it’s two Muirhead & Co. Ltd. Type A-2-B rheostats.
If that name sounds familiar to you, you may know it from its aerospace connections – but Muirhead & Co. Ltd. began as a humble telecommunications test equipment manufacturer.
The first thing to note is the heft of the handle and the pointer scale. The two units, being just one serial apart (sequential units) of the same type have different scale markings as if it was etched just for that particular rheostat – as a consequence of the tolerances in manufacturing.
Another thing to note is that these rheostats are housed in a die-cast box, with a plastic (acrylic possibly) front panel. That may have been a build to make the supplied Type A-2-B rheostats usable standalone. The three standard rheostat terminals are broken out to binding posts, along with a safety earth in the top right corner.
A closer look at the dial reveals the company name in its full glory …
… the type and the serial number designation.
Popping one of them open reveals the rheostat units housed in an aluminium tin can which is connected to the safety earth (by an unusually thin wire). The wires to the inputs/outputs are very thick, likely to reduce the resistance impact of having the connection leads – the resistances involved by the rheostats are so tiny (~0.1 ohm)!
It looks like these may have been used as a direct resistance to put in circuit as a shunt to measure current flow. A potentially dangerous practice if done without care – the low resistances and large wires inside suggest this may have been one of the applications.
The tin can is only attached by friction, and falls off with ease. Inside, we see thick copper and (possibly) nichrome wire wrapped around a former, with a wiper arrangement to form the resistance. Normally, we think of wire as having negligible resistance – but when you’re building a “variable negligible resistance”, it makes sense to use wires. The current handling capacity is probably quite high, but I have no data on this unit.
A quick close-up on the wipers …
It’s often interesting to see companies manufacture things you didn’t expect. Maybe I’ll stumble upon a Lamborghini tractor ..