Measurement of Resistance Questions and Answers

Answer: A

The conventional division puts everything at or below about 1 ohm in the low class, from 1 ohm to roughly 100 kilohms in the medium class, and everything above 100 kilohms in the high class. The boundaries matter because each class demands a different measuring arrangement.

Answer: B

A pair of screw terminals and a length of connecting wire contribute a few milliohms each. Against a 0.001 ohm specimen that is not a small correction but the greater part of the reading, so the potential connections must be brought out separately from the current ones.

Answer: C

The outer pair carries the measuring current and the inner pair taps the potential difference across a defined length of the element. Since the potential leads feed a high-impedance detector they carry virtually no current, so their own resistance and the contact drops at their junctions do not enter the result.

Answer: D

Cable insulation runs to hundreds of megohms and sometimes to thousands. An instrument shunt is a few milliohms, an armature winding a fraction of an ohm and a shunt field winding a few hundred ohms, which places them in the low and medium classes respectively.

Answer: A

Ohm's law is applied directly: the voltmeter reading divided by the ammeter reading gives the resistance. The method is quick and needs no special apparatus, but it is only as accurate as the two instruments together, so it is used where a few per cent will do.

Answer: B

The ammeter carries the current through the resistance plus the current drawn by the voltmeter. Since the denominator of the ratio is too large, the computed resistance comes out low. In effect one measures the unknown in parallel with the voltmeter resistance.

Answer: C

The voltmeter now reads the drop across the series combination, so the ratio gives the unknown plus the ammeter resistance. The error is therefore a known constant and can be subtracted. This connection suits a high unknown, where the ammeter resistance is negligible against it.

Answer: D

For a low unknown the voltmeter current is a small fraction of the total, so that connection errs little; the ammeter resistance, on the other hand, would be a large fraction of a low unknown and would spoil the other connection. The choice is reversed for a high unknown.

Answer: A

The unknown is connected, the current noted, and then a calibrated variable resistance is put in its place and adjusted until the same current flows. The standard then equals the unknown. Because the supply and the instrument are common to both readings, their errors largely cancel.

Answer: B

With arms P, Q, R and S taken in order round the bridge, balance requires PS equal to QR, so the unknown is obtained as the ratio P/Q multiplied by the known arm. The condition is independent of the supply voltage, which is the great merit of every null method.