( 215 ) 



ïn order to express ?/?« in tlie niilliiiicli'c-nilcranipere uiiils for a 

 photogram of which the speed of the sliding frame is Vu nmi. per 

 second, nii being the known vahie for a speed F^g, we use the 

 relation : 



m. = 7n,3 X f ^ J ■ • (8) 



From what precedes we see that the measurements required for 

 a calcidation of the value of ///- when the value of F is known, 

 are limited to : 



the sensitiveness c, 



the deflections «j, «^ . . . etc. and 



the period 7\ 



The measurement of the sensitiveness c presents no difficulties. 



The intensity of the current is known from the electromotive force 

 of the source and the resistances used. The source consisted of 

 storage cells, the electromotive force of which remained very constant 

 and might be put at 2 Volts, while the resistances were taken either 

 from manganin resistance coils or from a graphite resistance of 

 Siemens and Halske which had previously been checked. 



The deflection of the quartz thread is measured on the photogram 

 on which the network of square millimetres is found. With the 

 magnifying glass it is easy to estimate 0.1 mm., so that a deflection 

 of 30 mm., which is often used, is known with an accuracy of 0,37o- 



If the permanent deflection is u mm., the intensity of the current 



u 

 = i micrampere, then the sensitiveness is c = -r millimetres per 



I 



micrampere. 



The values of the oscillations «i, «„ etc. are, like the permanent 

 deflection, read off directly from the network of square millimetres. 

 But as these values are smaller than 'a and the absolute error in 

 each measurement remains, unchanged = O.i mm., the accuracy of 



the value found for ^ = — = — .. . etc. is not great. Moreover a 



«, «8 



a ex 



distinct difference is often found between — and — , so that we are 



«„ «, 



obli 



ged to calculate a mean value, e.g. by putting k =zly^ —. 



Fortunately the value of k, as it occurs in our measurements, has 

 only a small influence on the final result, relatively large variations 

 in k scarcely causing any difference in the calculated value of m. 



