A. A. Michelson—Rate of Tuning-forks. 63 
rupted the circuit of the induction coil. The results thus 
obtained left nothing to be desired. 
In order that the rate of EUt, remain constant, it is necessary 
that the current passing through its electro-magnets be con- 
stant. Hence the necessity for.constant batteries, which, how- 
ever, have a comparatively great resistance. Several might be 
joined in parallel circuit, but it was found that much better 
results were obtained by using six “gravity” cells in series, 
and using fine wire magnets. Such a battery will keep the 
fork in vibration indefinitely with but little alteration in its 
rate except that due to variations in temperature. 
In comparing the forks a convenient plan is to weight the 
KUt,, so that it gives with the fork to be rated about 9 beats 
in 10 seconds, and then to time the “coincidences” between 
the beats and the ticks of the pendulum. By this means the 
comparison can be much more accurately determined than by 
counting the beats for sixty seconds. This may also be done 
by the optical method. It was found that the “gravity” bat- 
tery which kept the EUt, in vibration could serve at the same 
time to work the relay. 
é whole arrangement is shown in fig. 1, in which, for 
clearness, all the parts to the right of ¢ have been shown in the 
plane of the drawing, instead of at right angles 
pendulum, | 
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In practice only the even flashes are counted, the odd ones 
being disregarded. Consequently, if a be the number of these 
per period; n, the rate of the fork, and 2N, the whole number 
hearest to 2n, then n==N+ Fa" To determine which sign is 
to be used, EUt, is weighted by a very smal] piece of wax. If 
@ 18 greater than before, the sign is +; if a is less, the sign is — . 
The electric fork may be dispensed with entirely by the 
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following method, The fork to be rated is placed vertical, so 
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