[Boatanchors] series light tester info needed

[Will]

Can't find the schematic for a series light tester in any of my radio books, nor by searching under that designation on the web. 

I need just the briefest description of how to construct and use said device, the kind of tester (sort of like a kludg-ey Variac)that accomplishes the purpose of gradually applying higher and higher voltage to a radio or other appliance of unknown condition, by means of increasing (decreasing?) the wattage of a light bulb used in series with the thing being tested. Thanks much!

DE WILL/KD7BFX