Linking Lower Voltage and Higher Voltage systems?

[dpsciarrino]

Linking "Lower" Voltage and "Higher" Voltage systems?

I'm an undergraduate in EE (entering sophomore level) and I have been wondering about how a low voltage control system is connected to a relatively higher-in-voltage system. (An example would be how a thermostat turns off and on a heater or air conditioner) I'm just wondering about the actual physical implementation, like what kinds of circuits are used to connect these two areas of one system.

 

[russ_watters]

Relays/contactors:
http://en.wikipedia.org/wiki/Relay
http://en.wikipedia.org/wiki/Contactor

Basically, a solenoid that throws a switch.