phy電學

Use a "Wheatstone Bridge".

The wheatstone bridge consists of four resistors, of which three are of known resistances and one unknown. They are connected in a "bridge form". That is, two branches of series connection joined together in prarllel, with a sensitive galvanometer connected across the bridge. The "bridge" is then connected to a battery.

By adjusting the known reistors, the bridge is made to balance by observing that no current flows through the galvanometer. The value of the unknown resisotr Rx is then given by,

Rx = (R2/R1).R3, where R1, R2 and R3 are the resistances of the three known resistors.

For detailed theory of the Wheatstone bridge and the connection with the reistors, please refer to the following web-page:

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