Kelvin bridge
The Thomson Bridge, also Kelvin Bridge, is an electrical bridge circuit down is used to measure small resistance values in the range of some 10 μΩ. It is under construction related to the Wheatstone bridge and is based on the four-wire measurement unwanted contact resistances, which falsify the measurement result to minimize. The circuit is named after William Thomson, 1st Baron Kelvin.
Has the Thomson Bridge, because of the expense associated with the manual adjustment of the bridge, in practical electrical metrology only of minor importance and is by digital ohmmeters, been based on the four-wire measurement, replaced with a reference resistor.
Construction
The circuit is as shown in the right sketch depicted, from the unknown and to be determined resistance Rx and a known resistance value in the reference resistor Rn. The to existing series resistors RL1, RL2 and RL3 make unwanted but inevitable line resistances, contact resistances of terminals and similar longer represents, are worth unknown and can also be greater than the to be determined resistance of Rx with small ranges in the resistance value to be. To avoid the adverse influence of this line and contact resistances, the so-called four-wire measurement is used in both the resistance to be determined as well as the reference resistor. The reference resistor can be designed as a shunt resistor with four ports. In this case, the current path in which a high Spannungsabsfall according to the contact and lead resistances occurs, separate from the voltage path with only small currents. The current path is supplied by an external voltage source U.
The stress paths provide the actual measuring bridge, consisting of the four resistors R1, R2, R3 and R4. In the bridge path to a voltmeter V is the displayed value must be adjusted as a matching condition of the circuit to the voltage value of 0V. The balance of the bridge is done by changing the four resistors R1, R2, R3 and R4, which change R1 to R3 and R2 to R4 same percentage. These two resistors are each performed mechanically in the form of two potentiometers, where R1 and R3 can be adjusted together, as well as R2 and R4. This Potentiometerbauform comprising two variable resistors is referred to as double potentiometer. Alternatively switchable voltage divider can be used for matching rather than double potentiometer, in which case in addition to the reference resistance R needs to be changed.
The bridge is balanced, this will be the case if the voltage indicator shows the meter 0 V, the following condition applies:
And the resistance value Rx has to be determined then, with known values of Rn, R1 and R2, the following value:
The unwanted line and contact resistances RL1, RL2 and RL3 not go here into the measurement with.