Voltage

I'l guess at what you mean -

1. If you only want to raise a few volts at line frequency, look at the transformers supplying this circuit, some transformers have "taps" typically in +/- 2%, or +/- 5%.

2. If that is not what you mean but still on a line frequency, then a "Variac" can adjust the voltage from zero to about +10% of line voltage, there are units available up to and greater then a Kilowatt. Ask and I can provide more info.

If you want something else, please provide more detail about what you are trying to accomplish ?

By transformer, by amplifier, by resonance.

Reminds me of a labourer on the shop floor asking me: "what is better, a transformer or a transistor?"

Same bright spark put a sun roof in his car. Instructions explicitly said to have the opening at the back, but he wanted to scoop the air into the car. After a weekend of mods he ran his car with his front opening sun roof down the highway at 100 clicks, opened the hatch and watched it disappear.