Measurement of Low Resistance - GATE EE Electrical and Electronic Measurements

The contact and lead resistances form a part of the circuit whose resistance is very high. As a result the contact and lead resistances are usually neglected compared to the high resistance value.

Kelvin’s double bridge is used for the measurement of low resistances of the order of 1ῼ or less. Ammeter voltmeter method is used for the measurement of current flowing through and the voltage across the circuit.

Low resistance refers to resistance of the order of 1ῼ or less than that. Medium resistances range from above 1ῼ to a few kῼ. Any resistance value greater than a few kῼ is known as high resistance.

When measuring low resistances of the order of 1 ῼ or even less, lead and contact resistances of the order of even 0.002 ῼ cannot be neglected. High currents flow through low resistance circuits.

Fig 15.1, illustrates the construction of low resistance. A is an ammeter used to measure current through the circuit, while V is the voltmeter used to measure voltage.

The voltage drop measured across a low resistance does not contain any contact and lead resistances of the components and is independent of it.

As the current flowing through a low resistance circuit is low, the voltage drop across the terminals due to contact and lead resistances is negligible. Thermal e.m.f occurs in a circuit when its temperature increases due to high current flow.

The terminals aa’ are used for the measurement of the voltage drop across the resistance R. An voltmeter V is connected across the terminals aa’.

By applying Kirchhoff’s current law at the node N from fig we get,
I = I_V + I_r
where,
I is the total series current flowing through the circuit
Iv is the current flowing through the voltmeter
Ir is the current flowing through the resistance R.

A simple bridge circuit is made of a network consisting of 4 resistance arms. Usually a galvanometer is connected between the ends of the opposite two junctions in order to measure the flow of current.