An easy way to generate a higher voltage from a low voltage DC source is with an inductor. Depending on the number turns of wire in the coil a 6 volt battery could produce a jolting 200V plus from a simple coil of wire.
Does inductance change with voltage?
Mutual-inductance: Mutual inductance is an inductive effect where a change in current in one circuit causes a change in voltage across a second circuit as a result of a magnetic field that links both circuits. This effect is used in transformers.