“Tesvac” vacuum tester, made by Ferranti Limited in the United Kingdom, 1950-1970. With original box, but without probe.

Scientists often do experiments under vacuum. One reason for this is to stop the experiment from being contaminated. Ferranti’s “Tesvac” was used to locate leaks in glass vacuum equipment used for scientific experiments. The glass vacuum system to be tested would be pumped full of safe helium gas, to see if there were any leaks. This Tesvac is missing its probe, but the probe would produce a continuous, high frequency spark of electricity from a small Tesla coil. If the glass vacuum system had a leak, then the helium gas inside would contact the electric spark produced by the probe. This would cause the helium gas inside the glass vacuum system to glow a purple colour, confirming that the vacuum system had a leak.