Electrical Engineering ⇒ Topic : Measurement of Low Resistance

Measurement of Low Resistance

Resistances of about 1 Ω and less are included in this class. Measurements of low resistances are required for determination of resistances of armatures and series field windings of large machines,ammeter shunts, cable length, contacts and so on.

The Kelvin's double bridge is one of the best for the precise measurement of low resistances.It is based on the Wheatstone bridge in which the errors due to contact and lead resistances are eliminated. The schematic diagram of Kelvin's double bridge is shown in Figure (a).

                                         figure (a)  Kelvin's double bridge

Let us assume that X is unknown resistance, S is standard resistance of same order of magnitude of X, and r is connecting link of low resistance.Q, M and q, m are the two pairs of known non-inductive resistances, and one pair out of Q,q or M, m is variable (for the purpose of balancing the bridge).

With large currents as mentioned earlier, there is often difficulty in ensuring that all the voltage drop occurs along the body of specimen and the contacts carrying current in and out of the specimen. It is very difficult even with soldered leads to avoid appreciable resistance in the contacts between a specimen and the leads connected to it. An arrangement shown in

Figure (a) is often adopted where the voltage measurement is isolated from the current leads and only small currents flow through the voltmeter. The arrangement also helps to fix the points between which resistance is to be measured.

Reducing the delta of q, m, and r to star circuit of a, b, and c shown in Figure (b), the balance equation of the resulting Wheatstone bridge can be written very easily.

                              Figure (b) Kelvin's double bridge

(Q/M) (q/m) is generally adjusted as closely as possible to balance the bridge. Also by making the value of r as small as possible, the value of resistance mrl (q + m + r) can be made small indeed. Thus, at balance,

Therefore, the balance condition is actually

                                                                                     ......... (1)

So, this bridge is called Kelvin's double bridge.

For Measurements of low resistances:-

1. Ammeter voltmeter method
2. Kelvins double bridge method
3. Potentiometer method