Measurement of Resistance Questions and Answers

Answer: C

Moisture lays a conducting film over the surface, and the current along that path is added to the wanted volume current so the computed resistance comes out low. A guard electrode intercepts the surface current and returns it to the source without letting it reach the detector.

Answer: D

Two readings are taken with the same supply and the same instrument, and the standard is adjusted until they agree. Any scale error or supply drift affects both readings alike and therefore drops out, leaving the accuracy of the standard as the limiting factor.

Answer: A

A single degree changes a copper resistance in the fourth significant figure, so a five-figure value without a temperature is not a statement about anything. Standards are made of manganin precisely because its coefficient is small, but the specimen under test usually is not.

Answer: B

Balancing a specimen of hundreds of megohms would need standard arms of the same order, which are neither stable nor readily made, and the current available for the detector would be minute. The megohm bridge with its guard circuit and high voltage supply is built for the purpose instead.

Answer: C

A cell drives current through the specimen and a series-limiting resistance, and the movement deflects accordingly. Because the reading depends on the state of the cell the scale must be zeroed before each use, and being a deflection method it is far less accurate than any bridge.

Answer: D

The current is inversely proportional to the total resistance, so the divisions crowd together as the resistance rises. Zero ohms gives the largest current and therefore full deflection, while infinite resistance gives no current and the pointer stays at the mechanical zero of the movement.

Answer: A

With the unknown in parallel with the movement a low resistance diverts most of the current and the pointer falls, so the scale opens out at the low end. The zero of such an instrument is at no deflection and the infinity mark at full deflection, the reverse of the series type.

Answer: B

The additions are exactly what high values demand: a high test voltage to produce a measurable current through hundreds of megohms, and a guard terminal to keep surface leakage out of the detector branch. The balance principle itself is unchanged.

Answer: C

The loss of charge result is proportional to the time divided by the capacitance, so an error in the capacitor is an equal error in the answer. A bridge needs no such quantity: only the ratio of its arms and the value of its standard enter the balance condition.

Answer: D

Twenty degrees is two steps of ten, so the resistance is halved twice, giving a quarter of the original: 200 megohms. This steep dependence is why insulation readings are always corrected to a stated reference temperature before they are compared with earlier records.