You don't see many 667-A inductance bridges around, though they must have been plentiful at one time. Mine was purchased at a hamfest in Horseheads NY a number of years ago for less than $50, including a manual. The date in the manual is 1953.
The 667-A was designed to measure the inductance of coils having low storage factor (Q) at audio frequencies. It's somewhat peculiar in that it isn't recommended for measuring Q, nor does it read it out directly. Q can be calculated from the dial readings, but GR recommends against it due to various error sources. The direct measurement range is 0.1 uH to 1 H. The bridge will balance for any Q from 0.6 to infinity. Accuracy is 0.2% and the frequency range is 60 Hz to 10 kHz.
This style of inductance bridge suffers from non-unique balance that is, the controls interact making determination of the true balance point difficult. GR cleverly got around this problem by including a variable inductance (variometer) in series with the unknown. By taking the difference in variometer readings at the start and end of balance, the value of the unknown is obtained in a straightforward manner. Well, at least compared to what you'd have to do without the variometer.
GR also compensated the resistance decades for constant inductance within 0.1 uH at any setting.
This is a very serviceable bridge and I suspect many ended up in the hands of hams for checking small RF inductors. I find that that balance is easy to achieve if you already know the approximate value of the inductor. If the value is completely unknown, it takes a little longer, since there may be no clue that balance lies in a particular direction.
The 667-A is the predecessor to the "standards lab" 1632-A inductance bridge. Once the 1632-A was available, there was probably little call for a inductance bridge that couldn't read Q, plus the 1632-A had a six digit readout. The coming multifunction bridges like the 1608 and 1650 could read C, G, Q, R & L, all in one unit, and would also have pushed the 667-A off the market. In spite of that, the 667-A is a nice looking instrument with its black face and wood case. It's a pleasure to use and reminds me of what engineering might have been like in a slower time.
C. Hoffman
last edit October 11, 2011