A made-to-order application that uses most of the features mentioned above is the inter‑comparison and scaling of inductance standards. This application is particularly important because there has bean no inductance bridge available with ppm resolution except, maybe the GenRad 1632 which was discontinued several years ago. The 1632 was a six‑digit, two‑terminal bridge (one grounded) with only 0,1 % direct‑reading accuracy. It was very good for comparing two, similar decade‑valued (1, 10, 100 etc) inductors. It wasn't particularly good by today's standards because of the 0,1 % direct reading accuracy. Balancing a 1632 to high precision was a slow procedure especially when measuring standards like the QuadTech 1482 Inductance Standards which have low Q values, especially at 100 Hz , where many measurements must be made. Moreover, the effective ac resistance of these standards is primarily the resistance of copper wire which has a temperature coefficient of almost 4000 ppm/degree C. Even a 1/100th of a degree change in the temperature of the wire due to ambient changes or applied power causes an annoying bridge unbalance which makes the inductance measurement difficult. An automatic bridge like the QuadTech 1689 has no such problem; the resistance can change at any (reasonable) rate without affecting the inductance reading (if you measure equivalent series inductance). Because of this, and the 1689 ease of use, the instrument is ideal for making 1:1 comparison measurements on inductance standards.

A five‑terminal (guarded and Kelvin) capability would have been an advantage in calibrating these inductance standards, but because they were being calibrated long before either 3‑terminal or 4‑terminal capability was available on an inductance bridge, NIST uses a grounded, 2‑tenninal connection. With early precision meters, ground was guard, not one of the main connections, but this did not cause a problem. The only thing to remember, when measuring something like the QuadTech 1482 Standard Inductor is to tie the case to its LOW terminal (with the link provided) and to insulate the case from ground. This keeps the internal stray capacitance’s that effect high‑inductance measurements the same as the 2‑terminal calibration by NIST.

Standard inductance’s are particularly hard to scale in value by combining two or more units in series or parallel (as is done with resistors) because of their size, their low Q values and the stray capacitance’s involved when two are connected together. Transformer‑ratio‑arm bridges, capable of precise scaling, are not available for inductance measurements. They can be made from commercially available parts, but are difficult to construct and use. Fortunately, extreme accuracy is not required because the best NIST calibrations have an estimated uncertainty of only 0,02 %. However, ratio measurements used to scale these calibrations should be much tighter to avoid adding errors. Ratio measurements of 2:1, made on a 1689, on the same range and using the ∆ % readout, typically have errors totaling less than 20 ppm. This allows comparisons of inductors of intermediate value (those values starting with a 2 or 5) to be compared against the even decade values with negligible added error. Scaling calibrations over a 10:1 range is less accurate because a range change is often required, or if on the same range, there is apt to be increased non‑linearity error. They should be good to 50 ppm at 1 kHz over the basic overall range of the instrument.