Bridge circuits in RTDs - why are they there? how do they work?

Question

Why would it have to have 4 sides to the circuit? Are they comparing resistance in one arm to that in another? If the two arms don't have the same resistance, what happens? Why do they have one variable resistor and one resistor with an undefined value? I know that heat varies resistances in the materials used, so that an RTD is an effective temperature/heat indicator. But what I've read about how the bridge circuit that measures the current/resistance doesn't make sense to me. Could someone explain in non-technical terms (I'm new to engineering) how the bridge circuit works in an RTD, and the purpose of each arm? Thank you for your help!

Answer 1

The point of the bridge is to generate a voltage large enough to be used for display etc from a small change in resistance. Often its used in load cells as the resistance change is very small. An RTD would normally give a much larger change but it would work the same.

http://en.wikipedia.org/wiki/Wheatstone_...