1881.] Determination of the Ohm in Absolute Measure. Ill 



method rather long spinnings (ten or twenty minutes) are necessary 

 in order to get the speed with sufficient accuracy, much longer than 

 are required to take the readings at the telescope. Desirous, if 

 possible, of making the observations more quickly, I determined to 

 try the stroboscopic method. On the axis of the instrument a stout 

 card of 14 inches diameter was mounted, divided into concentric 

 circles of black and white teeth. The black and white spaces were 

 equal, and the black only were counted as teeth. There were five 

 circles, containing 60, 32, 24, 20, 16 teeth respectively, the outside 

 circle having the largest number of teeth. 



This disk was observed from a distance through a telescope, and an 

 arrangement for affording an intermittent view. An electric tuning 

 fork of frequency about 63-^ was maintained in regular vibration in 

 the usual way by means of a Grove cell. To the ends of the prongs 

 are attached thin plates of metal, perforated with somewhat narrow 

 slits parallel to the prongs. In the position of equilibrium these slits 

 overlap so as to allow an unobstructed view, but in other positions of 

 the fork the disk cannot be seen. When the fork vibrates, the disk is 

 seen intermittently 127 times a second; and if the speed be such that 

 on any one of the circles 127 teeth a second pass a fixed pointer, that 

 circle is seen as if it were at rest. 



By means of the various circles it is possible to observe correspond- 

 ingly varied speeds without any change in the frequency of the fork's 

 vibration. A further step in this direction may be taken by modify- 

 ing the arrangement for intermittent view. If the eye be placed at the 

 top or bottom of one of the vibrating plates, a view is obtained once 

 only, instead of twice, during each vibration of the fork. This plan 

 was adopted for the slowest rotation, and allowed 60 teeth to take 

 the place of 120, which would otherwise have been necessary. 



The performance of the fork was very satisfactory. It would go 

 for hours without the smallest attention, except an occasional renewal 

 of the alcohol in the mercury cup. Pure (not methylated) alcohol was 

 used for this purpose, and a platinum point made and broke the 

 contacts. Although, as it turned out, this fork vibrated with great 

 regularity, dependence was not placed upon it, but repeated compari- 

 sons by means of beats were made between it and a standard fork of 

 Koenig's construction, of pitch (about) 128. These beats, at pitch 

 128, were about 48 per minute, and scarcely varied perceptibly during 

 the course of the experiments. They could have been counted for an 

 even longer time, but this was not necessary. It was intended at 

 first to make the comparisons of the fork simultaneous with the 

 other observations, but this was given up as a needless refinement. 



Some care was necessary in the optical arrangements to obviate 

 undue fatigue of the eyes in a long series of observations. In daylight 

 the illumination of the card was sufficient without special provision, 



