Old Scientific Instrument

This beautiful old instrument is a Callendar-Griffiths Bridge.

It is one of many similar old instruments, many in fine mahogany cases, that were lying around our department, and in principle destined for the skip. This is one I decided to rescue!

Such things were evidently constructed with loving care, and it's a real shame that they are unwanted. They aren't rare enough for museums to accept them, because they already have examples, and they have long since been superseded by small devices that are much easier to use (and that are probably more accurate, especially in inexperienced hands). All that said, I have just found a similar bridge on eBay with a current asking price of £1300! I wonder if it will sell.

For those interested in the device as an instrument rather than as a thing of beauty, here is what it's for. It is a modification of the famous Wheatstone bridge, specifically designed for measuring the resistance of the probe in a platinum resistance thermometer. It is thus a step towards measuring temperatures more accurately than is possible with the simple Wheatstone bridge.

I've just found a Museum site where many of the instruments similar to those that I have 'liberated' are described. Indeed, quite fortuitously, my photograph of today shows in the background one of these Avo 8s (said to be worth a staggering £500!).