AC Bridges Questions and Answers

61.

A bridge is balanced at 1 kHz and the supply is then changed to 2 kHz without touching the controls. The detector remains silent. This shows that

A.the balance equations contain no frequency term
B.the detector has been left tuned to the original frequency of one kilohertz
C.the arms of the bridge are all pure resistances with no reactance whatever
D.the supply contains no harmonics at either of the two frequencies employed

Answer: A

Only a bridge whose balance is frequency independent can hold its null when the frequency is doubled. Maxwell's inductance bridge and the simple De Sauty bridge behave so; a Wien bridge would lose its balance at once, which is what makes it a frequency meter.

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62.

The chief reason a Wheatstone bridge cannot simply be used with an alternating supply to measure an impedance is that

A.the products of the opposite arms are no longer equal when the supply alternates
B.it provides only one adjustment where two are needed
C.the alternating current would heat the arms and change the values being compared
D.a galvanometer cannot be used as a detector on an alternating supply at all

Answer: B

The balance rule itself carries over unchanged; what does not carry over is the sufficiency of a single control. An impedance has magnitude and angle, and a bridge with one variable resistance can satisfy only the magnitude part of the condition.

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63.

In a bridge measuring the capacitance of a long cable, the sheath is earthed and the measurement is made between core and sheath. The Wagner earth is then

A.unnecessary, since the capacitance of a cable is far larger than any stray present
B.harmful, because it would place a second earth on a circuit already earthed once
C.still needed, for the strays of the other arms
D.unnecessary, because the specimen is already earthed at one of its two terminals

Answer: C

Earthing the specimen deals with one stray path only. The remaining corners of the bridge, the leads and the detector all retain capacitance to earth, and the Wagner branch is what keeps those out of the balance. Nothing prevents both earths existing, since the Wagner point is at earth potential.

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64.

A dissipation factor of 0.02 corresponds to a loss angle of about

A.2 degrees
B.0.02 degrees
C.88.9 degrees
D.1.15 degrees

Answer: D

For small angles the tangent and the angle in radians are nearly equal, so the loss angle is about 0.02 radian, which is \( 0.02 \times 180/\pi = 1.15 \) degrees. The phase angle of the specimen is the complement of this, about 88.85 degrees.

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65.

The quantity actually held constant in a Schering bridge as the test voltage is raised is

A.nothing, both readings being retaken at each voltage
B.the dissipation factor, which is a property of the material and does not vary with stress
C.the capacitance of the specimen, which is fixed by its geometry alone
D.the setting of the adjustable arms, the changes being read from the detector deflection

Answer: A

A tip-up test consists precisely in rebalancing at each of a series of voltages and plotting how the loss tangent grows. Both capacitance and loss tangent can change as voids begin to discharge, so neither may be assumed constant, and the arms are reset at every step.

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66.

An alternating bridge is being used at 10 kHz and the leads to the specimen are a metre long. The likely difficulty is

A.the standard capacitor losing its calibration above a few kilohertz
B.the inductance and capacitance of the leads themselves
C.the resistance of the leads
D.the ear being unable to judge a null at ten kilohertz with any accuracy

Answer: B

At ten kilohertz a metre of wire has an inductance of a microhenry or so and a capacitance to its neighbours of tens of picofarads, both of which enter the arm they belong to. Precision work at such frequencies uses short screened leads and allows for what remains.

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67.

The purpose of an intermediate transformer between the source and an alternating bridge is to

A.filter the harmonics out of the supply before it reaches the arms of the bridge
B.match the impedance of the source to the input impedance of the bridge network
C.isolate the bridge from earth through the supply
D.step the voltage up so that a larger unbalance signal reaches the detector

Answer: C

Without isolation the supply itself provides an earth connection at some point in the network, which puts stray capacitance across the arms in an uncontrolled way. A screened isolating transformer leaves the choice of earth point to the operator, which is what makes a Wagner earth possible.

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68.

Comparing the simple De Sauty bridge with the Schering bridge, the essential addition in the latter is

A.a second standard capacitor placed in series with the specimen under test
B.a variable inductance which balances out the loss angle of the specimen
C.a tuned detector
D.a parallel resistance and capacitor forming an adjustable arm

Answer: D

That parallel combination is what allows the loss of the specimen to be balanced, and its two components appear together in the expression for the dissipation factor. The high test voltage and the earthing of the low-voltage arms complete the difference between the two bridges.

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69.

A bridge arm consists of a resistance in series with an inductance. Its phase angle is

A.positive and less than ninety degrees
B.negative
C.exactly ninety degrees
D.zero

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.

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70.

A bridge arm consists of a capacitor with a resistance in parallel. As the resistance is increased without limit, the phase angle of the arm tends to

A.minus forty-five degrees
B.minus ninety degrees
C.plus ninety degrees
D.zero

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.

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