@David: Actually, you would go about it a bit differently. Believe me- you wouldn't want to be adjusting one of these while voltage is applied.
Assume for a moment that you need to run a hipot test on a winding in a generator at a hydroelectric dam. If you allow the voltage to go too high you could damage the extremely expensive generator. You start by determining the test voltage you want to apply. Then you consult the tables and set the sphere gap accordingly. You will also have an analog meter on the hipot test set so you have two meters for safety. You run the voltage up until the sphere gap establishes an arc. This not only verifies that you reached the desired test voltage, but it assures that you can't go any higher.
To do the initial setting you just use an accurate hand-held scale. There is a small scale on the bottom of the sphere gap, but it's a relative scale so you can tell that you opened or closed it a certain number of millimeters from the original setting.
Rick, I appreciate that you measure the voltage by bringing the spheres together until you get arcover, but how do you read the voltage off these things? Is there a scale in the bottom mount above that four pronged turning handle? Then you read that and plug the value into yoru tables?
I guess I'd better drop the other shoe- The devics in the photo are voltmeters. More specifically, they're Sphere Gap Voltmeters. The big one in the picture has spheres that are 6.25cm in diameter and can be used to measure up to 78KV with an accuracy of 3%. To get this kind of accuracy you have to consult tables that take into consideration the temperature, humidity, and atmospheric pressure. The cylindrical devices like the one the model is holding are current limiting resistors, so when the gap flashes over you get a "dzzzt" instead of a KABOOM!
This may be old technology but these are still being used today. There's not a whole lot that can go wrong with these. No worries about a sticking pointer, like on an analog meter, and you certainly don't have to worry about the batteries running down, as in a digital meter!