258 Prof. C. Y. Boys on Photographs of Rapidly Moving 



C. 

 U 



Outside diam. 

 . 12 

 . 1H 



Inside diam. 



84 



7* 



Depth. 

 6 



The resistances of the coils are : — 



C 

 U 



B.A. units. 

 2-423 

 2-514 



Ohms. 

 2-391 

 2-481 



The self-inductions determined by Sumpner's method 

 (Journal of the Soc. of Telegraph Engineers, 1887, fig. 5) 

 were found to be 



C 



u 



26-89 x 10 6 C.G.S. units. 

 26-31 x 10 6 



In comparing the number of oscillations measured with 

 those calculated from the electric constants given, experience 

 showed that the photographs, such as those in fig. 6, were not 

 so suitable as one taken with the spark in the line of the axis 

 of the wheel of lenses, because, though the definition along a 

 radius was not so good in the latter arrangement, it was for 

 the purpose of determining the intervals of darkness just as 

 good, moreover it gave a much larger arc on one plate which 

 made it possible to obtain the centre on the plate, and therefore 

 the angle between two points on the arc, with greater accuracy. 

 If the lenses were screwed into the face of a flat cone so that 

 the axes of each should meet in the axis of rotation at the 

 place of the spark, then definition would be equally perfect in 

 all parts of the circle. I did at first intend to place the lenses 

 in such a cone, but for various reasons the plate seemed pre- 

 ferable. 



The comparison of the calculated number of oscillations 

 w T ith the number found is complicated by the relations between 

 true ohms and B.A. units, as follows : — 



If the condenser is standardized with reference to the B.A . 

 unit of resistance, then, since the B.A. unit is T33 per cent, 

 too small, the number expressing the resistance is too large 

 and therefore the number expressing the capacity is too small, 

 since one varies inversely as the other, and so the number 

 expressing the capacity of such a condenser must be multi- 

 plied by 1/0133 to obtain the true capacity. 



The self-induction determined by comparison with capacity 

 and resistance is found from the expression 



L = a number x i\ x r 2 x C. 



If the resistances i\ r 2 are given in true ohms, then, as the 



