He’s using an armature growler (YouTube link) to test the armature from a generator. Armature growlers ... Still, if you work with DC motors or generators, it’s a great tool to have around ...

A M-G (Motor Generator) set was little more than a motor connected to a generator. The motor might take, say, 12V DC and the output could be, for example 300V AC that would get rectified for the ...