Transformer ratio arm bridge
The accuracy of this class of bridge depends on the ratio of the turns on one or more transformers.
A notable advantage is that normal stray capacitance across the transformer, including lead capacitance, may affect the sensitivity of the bridge but does not affect its measuring accuracy.
Subsequently the ratio arm principle was applied more generally, to other classes of electronic components and at frequencies up to r.f., and with many variations in how the unknown component was connected to the transformer or transformers.
[5] He made his first bridge while employed by the British company Standard Telephones and Cables, which did not manufacture test instruments.
TRA bridges have since been made by many specialist manufacturers,[1] including Boonton, ESI (formerly Brown Engineering and BECO), General Radio, Marconi Instruments, H. W. Sullivan (now part of Megger) and Wayne Kerr.
[7] (The two transformers allow both the signal source and the null detector to be isolated from the measured component.)
have the same value and are fed from the same tap on T1, the antiphase currents cancel out perfectly and the null detector will show balance.
An exact balance may be achieved by using two or more standards connected to suitable taps.
[2] The standards are shown as variable components connected to fixed taps on the T1 secondary, but bridges can equally be made with fixed standards connected to variable taps.
Also the numbers of turns on the two arms of the T1 secondary are not necessarily equal, and likewise those on the T2 primary.
Combinations of these various options offer great flexibility of construction, allowing measurements over a wide range of values while using only a small number of standards – essentially one per significant figure of the resistance or conductance value and one per significant figure of the reactance or susceptance value.
(Later versions of the instrument, with transistorised circuitry, used a moving-coil meter as the display for the null detector.)
Continuous ("vernier") fine adjustment to give third and fourth significant figures is provided by Rs3 and Cs3.
Similarly the polarity of the resistance standard can be reversed, so that measurements can be made in all four quadrants.
Besides the main balance controls described above, the front panel of the instrument has zero adjustments for both resistance and capacitance.
The inductive elements of the wire-wound resistance standards are compensated by trimming capacitors.
The susceptance reading is displayed as capacitance, and inductance must be calculated as a reciprocal using To simplify the arithmetic, the bridge operates at 1592 Hz so that ω2 is 108 s−2.
On the most sensitive ranges, readings must be adjusted to take account of lead resistance and inductance.
The external link allows two-, three- or four-terminal measurements to be made.
With additional external components, capacitors with a polarising voltage or inductors with a standing direct current can be measured.
An optional low-impedance adaptor extends the measuring range downwards by another four orders of magnitude, giving full-scale readings down to 10 mΩ, 5 F and 1 μH at ±1% basic accuracy.
Henry P. Hall, A History of Impedance Measurements, based on a draft for an unpublished book.
