Ohmmeter

The Kelvin bridge, also called a Kelvin double bridge and in some countries a Thomson bridge, is used to measure an unknown electrical resistor. What makes this device special is that it can measure resistors of less than 0.1 ohm. It is therefore an improvement over its precursor, the Wheatstone bridge, which was not able to do so. The Kelvin bridge was invented by William Thomson, who was knighted as Lord Kelvin.
The device works as follows. The bridge circuit has four resistors that are set to between 10 and 1500 ohms before performing the measurement. This is done by changing the wiring connections at the top and placing or removing the brass plugs. On the side is a lever that operates as a sliding contact resistor that controls another resistor with a very low resistance that can be varied between 0.01 and 0.001 ohms. The set resistance is read on a scale on the top of the device. The resistance to be measured is connected to the clamps at the edge of the baseplate. This results in a circuit containing six resistors: the unknown, four fixed, and one variable resistor. Finally, a voltmeter and a voltage source are connected. Changing the resistance of the variable resistor changes the voltmeter reading. When it is zero, the value of the unknown resistance can be derived from the values of the variable resistor and the four fixed resistors.