1881.] Determination of the Ohm in Absolute Measure. 131 



Lord Rayleigh. For simplicity we generally denoted the speed by 

 means of the number of teeth on the circle which seemed stationary 

 when looked at through the tuning fork ; thus we spoke of a speed 

 24 teeth, 32 teeth, and 60 teeth. To obtain the lowest speed the 

 circle containing 60 teeth was looked at over the top of the tuning 

 fork, so that only one view for each complete vibration was obtained ; 

 this was equivalent to a circle of 120 teeth in the ordinary arrange- 

 ment, which allowed a view for each half vibration, and, consequently, 

 the lowest speed was called 120 teeth. The velocity of rotation 

 depends, of course, on the frequency of the fork, which varied only 

 within narrow limits, and was always very near 63*69. If / denote 

 this frequency and N" the number of teeth on the stationary card, the 

 velocity of rotation w is given by the equation to=4irf/]$. In the 

 " British Association Report" the speed is always indicated by means 

 of the time occupied by 100 revolutions. If T is this time, we find 

 T = 50N//. The following table gives the comparison of w, T, and N", 

 on the supposition that the frequency of the fork was always the 

 same and equal to 63*69. 









Number of turns 





<x>. 



T. 



per second. 



120 



&670 . . 



. . 94-206 



. . . . 1-06 



60 .. 



. . 13-339 . . 



. . 47-103 



.... 212 



32 .. 



. , 25-011 . . 



. . 25-122 



. . . . 3-98 



24 .. 



. . 33-348 . . 



. . 18-841 



. . . . 5-31 



The last column gives the number of turns per second. 



Three speeds were taken on each of the three nights, and one set 

 of observations with the lowest speed was secured in the course of 

 one afternoon. We obtained in this way three sets for each of the 

 two intermediate speeds and two sets for the lowest and highest 

 speed. A comparison with the Report of the British Association 

 Committee shows that we do not go up quite to their highest speeds, 

 but that on the other hand our lowest speed was considerably below 

 the one used by them. In the Report for the year 1863, it is men- 

 tioned that the forced vibration of the magnet, depending on the 

 rotation of the coil, could not be noticed, and it is calculated that the 

 amplitude of this vibration was less than ^ of a millimetre on the 

 scale. No mention is made of this forced vibration in the Report for 

 1864, although much lower speeds were used during that year. In 

 our lowest speed a slight shake of the needle, due to the varying 

 action of the currents in the coil, was distinctly seen ; but as calcula- 

 tion showed that the amplitude was only the eighth part of a milli- 

 metre on the scale, no appreciable error is supposed to have been 

 introduced by it. The moment of inertia of the suspended parts was 

 higher in the experiments made by the British Association, and this, 



