Schering's bridge is used to measure
Answer: A
It is the standard bridge for insulation testing, giving both the capacitance of a specimen and its loss tangent at a test voltage of many kilovolts. The loss tangent is the more valuable figure, since it rises long before insulation actually fails.
Please log in or register to report this question.
In Schering's bridge the two low-voltage arms are kept at a low potential and earthed at one point so that
Answer: B
The bulk of the applied voltage falls across the specimen and the standard capacitor, which are the two high-voltage arms. The adjustable arms carry only a few volts, so their controls can be handled and the detector connected without hazard. Guarding and earthing then complete the safety.
Please log in or register to report this question.
The standard capacitor used in Schering's bridge is
Answer: C
The dissipation factor of the specimen is measured against that of the standard, so the standard must have a loss so small as to be negligible. Compressed gas capacitors achieve loss tangents of a few parts in a million and hold their value at high voltage.
Please log in or register to report this question.
The dissipation factor obtained from Schering's bridge is given by the product of the angular frequency and
Answer: D
Balance yields \( \tan\delta = \omega C_4 R_4 \), where the subscripted quantities are the capacitor and resistor forming the parallel adjustable arm. The dissipation factor therefore depends on frequency, so the test frequency is always quoted with the result.
Please log in or register to report this question.
A rising dissipation factor measured on a machine winding year after year indicates
Answer: A
Voids, moisture and contamination all increase the loss component of the current without changing the capacitance much, so the loss tangent climbs. Because it is a ratio it is independent of the size of the specimen, which makes it comparable from one test to the next.
Please log in or register to report this question.
The Wien bridge is chiefly used to measure
Answer: B
Its balance depends on frequency, so once the arms are set the bridge is silent only at one frequency. That property makes it a frequency-measuring network and, with an amplifier round it, the frequency-determining element of the Wien bridge oscillator.
Please log in or register to report this question.
A bridge whose balance depends on frequency can be used as a
Answer: C
Balance at one frequency alone is exactly what a frequency measurement needs, and putting an amplifier around such a network makes it oscillate at that frequency. Most bridges are deliberately designed so that their balance is free of frequency, which is the opposite requirement.
Please log in or register to report this question.
A bridge whose balance equations contain no frequency term has the advantage that
Answer: D
Maxwell's inductance bridge and the simple De Sauty bridge are of this kind. Frequency drift then does no harm and the result may be quoted without stating the frequency. Harmonics still trouble the null, however, since the residual arms are not perfectly reactance free.
Please log in or register to report this question.
The Wagner earthing device is used in a bridge to
Answer: A
A pair of impedances in series across the supply is adjusted until their junction is at earth potential, and that junction is then earthed. The stray capacitances of the bridge corners to earth are thereby placed across a branch carrying no detector current, so they drop out.
Please log in or register to report this question.
Balancing a bridge fitted with a Wagner earth requires
Answer: B
The detector is first switched to the earth point and the Wagner branch adjusted for a null, then switched back to the bridge and the arms adjusted. Each disturbs the other, so the two are worked alternately until both nulls hold at once.
Please log in or register to report this question.
