A Method for Determining the Rate of Tuning-forks. 85 

 by one tenth ; and in general if a flashes occur in one period, 

 then EUt 2 makes 128 + - vibrations per second. 



Since EUt 2 vibrates continuously, the number of periods 

 which may be counted is unlimited; hence a can be found 

 with any desired degree of accuracy. 



It was found that the chief difficulty in executing this plan 

 lay in the imperfections of the break-circuit. If thereby the 

 seconds intervals differed as much as 0*002 second, then the 

 operation would be impracticable, on account of the irregu- 

 larities in the position of the flashes. A great many break- 

 circuits were tried; and the one which was finally adopted as 

 being the most reliable and giving the least trouble was the 

 " mercury globule. " Even this, however, in its usual form 

 was unsatisfactory. A great improvement was effected by 

 narrowing the globule in the direction of the swing of the 

 pendulum, by placing it in a small tube whose upper end 

 was flattened. If the current from a "bichromate" cell is 

 interrupted by this break-circuit, the mercury oxidizes rapidly, 

 and the flashes (given by a small induction-coil through the 

 Geissler tube) become irregular. 



To obviate this difficulty, the circuit of a " gravity " battery 

 was interrupted; and this worked a relay, which in turn inter- 

 rupted the circuit of the induction-coil. The results thus 

 obtained left nothing to be desired. 



In order that the rate of EUt 2 remain constant, it is neces- 

 sary that the current passing through its electro-magnets be 

 constant. Hence the necessity for constant batteries, which, 

 however, 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 

 EUt 2 , 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 " 

 battery which kept the EUt 2 in vibration could serve at the 

 same time to work the relay. 



The whole arrangement is shown in fig. 1, in which, for 

 clearness, all the parts to the right of i have been shown in the 

 plane of the drawing, instead of at right angles. 



