AC Bridges Questions and Answers

Answer: A

The current lags the voltage in such an arm, so the impedance angle is positive, and it is less than ninety degrees because the resistance contributes a real part. Only a loss-free inductor would give exactly ninety, and a real coil never does.

Answer: B

With the resistance removed altogether only the capacitor remains, and a pure capacitance has an impedance angle of minus ninety degrees. A finite parallel resistance brings the angle back toward zero, which is how the loss of a capacitive arm is represented in the balance equations.

Answer: C

The simple bridge has resistances in two arms and capacitors in the other two, which satisfies the magnitude condition but leaves the phase condition unmet unless the two loss angles happen to be equal. Adding a series resistance to each capacitive arm, as Grover did, supplies the missing control.

Answer: D

At balance the result depends on the ratio of the arms and the value of the standard, all of which can be made accurate to a small fraction of one per cent. A deflection measurement can never be better than the instrument scales, which are good to perhaps half a per cent at best.

Answer: A

At a kilohertz headphones give a sharp null, whereas at power frequency the ear is nearly deaf, and the stray and residual reactances of the arms are still small enough to be manageable, which they are not at a hundred kilohertz.

Answer: B

The permeability of iron falls once the flux density approaches saturation, and the inductance falls with it. An iron cored inductance therefore has no single value, and the current or voltage at which it was measured must be quoted alongside it.

Answer: C

Both are models of the same physical behaviour at one frequency, so the ratio of loss to reactive current, which is the loss tangent, must be the same. The resistance in the series form is small while that in the parallel form is large, and each is related to the other through the loss tangent.

Answer: D

At a hundred kilohertz the residual inductance and self capacitance of a resistor are no longer negligible against its resistance. Arms for such work are wound in a way that cancels the flux, and are kept physically small so that the capacitance between turns is low.

Answer: A

The controls interact, so a setting made for the magnitude condition disturbs the phase condition and the other way round. Successive approximation converges quickly in a well-designed bridge; the merit of Owen's bridge is that its two conditions do not interact at all.

Answer: B

The two balance conditions yield the capacitance and the dissipation factor together, and the latter is the more searching figure. Insulation resistance is a separate direct-voltage test, and neither frequency nor temperature comes out of the balance, though both must be recorded.