My return to Classic Cmp - San Diego, software archive, etc.

It was quite common at one time. The ones I've memtioned have ther traditional H-shaped glass things in them (the Blackburn vavled one has a pair of them, just bare glass, on a bracket, the Solartron ones have them encased in a little plastic housing). I have a later DVM which has an enacpsulated thing with 2 wires comiong out that appears to be a standard cell, certainly the voltage between said wires, measured on a high-impedance votlmeter, is about right. On these instruments, there's a front-panel calibration cotnrol. You set the range switch to aparticular positon (and there's a warning about not leaving it there for logn periods, e.g. when you switch the instrument off), and adjust this control for the right reading. I have full service manuals for the Solartron instruments I have. I don't, alas, have anything on the Blackburn. As ever I have no canner or digital camera... -tony