| When I was an undergraduate in the second half of the
1950s, we spent a good deal of time studying inductance and how it was measured
with alternating current bridges. The power source was a 1000 Hz hummer,
and the detector was a pair of magnetic earphones.
Physicists of earlier generations used galvanometers for detectors, but the galvanometer is a direct current instrument. The solution was the Sechometer invented by Prof. W. E. Ayerton and John Perry in 1887. This consisted of a crank turning a shaft on which were connected two sets of contacts. One set of contacts turned the DC from a battery into AC, and the other took the resulting off-balance AC signal to be detected, and turned it back into DC.
The sechometer at the right was made by Herbert G. Dorsey of the Denison University class of 1897. In the last part of the nineteenth century Denison students regularly built apparatus as part of their experimental physics course.
|The device at the left is in the Garland Collection of Classical Physics Apparatus at Vanderbilt University. It is clearly some sort of commutator, and may be either be a sechometer or an electrotome.|
| The sechometer at the right
was made by the
#W. and L.E. Gurley company of Troy, New York. The firm was well-known for its fine surveying instruments, but made very few examples of sceintific instruments. This one is in the Greenslade Collection.
The crank can be placed on one of two drive shafts projecting from the near side, giving two speeds of rotation of the rotary swiches on both sides (to allow both terminals of the galvanometer and power supply to be switched back and forth.) The flywheel keeps the rotation rate fairly constant.
A very similar piece of apparatus was offered by Leeds & Northrup for $65 about 1910, while the 1896 Biddle catalogue sold another very similar item for $60, with the note that "it was recently designed by Messrs. Willyoung & Co., with the advice of Prof. [J.A.] Rowland [of Johns Hopkins University]." Within the year Willyoung sold out to Morris E. Leeds; Leeds & Northrup dates from 1903.