( 21? ) 



which in itself is not free from errors. The ordinates of the net 

 are recorded by a quickly revolving disk with spokes, casting 500 

 shadows per second on the photographic plate. The variations in 

 the rotational speed of the disk cause onl}' small errors. Reserving 

 a more extensive description of this time-marker for a later occasion, 

 we may here mention that the rotational speed is very constant and, 

 in all probability, always agrees to 0.1 "/„ with the pre-determined 

 value. But we have no guarantee that the mutual distances of the 

 spokes, although carefully marked off on the lathe, come up to the 

 same standard. 



The greatest errors are probably made by the setting of the cross- 

 wire. To be sure, surprisingly sharp settings can be made on the 

 fine, sharp top, representing the turning point of a movement of the 

 string, so that a greater error than Ifi need not be made, but often 

 it is necessary to mark the exact middle of a broader top ; in which 

 case the error becomes larger, of course. With many of our measure- 

 ments of the period T the error must be estimated at 1 to 27o- 



We shall now give the results of some measurements. 



With a speed of the sensitive plate of F= 100 mm. per second, 

 some photograms were taken of string n". 10. 



The sensitiveness w^as c = 10.92 mm. per micrampere, 

 the period T= 0.43 mm. 



the damping ratio /: = 3.3 



With these data the value of m is calculated from formula (7) at 

 3.76X10-^ [mm— ft.4]. 



For a comparison with following results this value is reduced by 

 formula (8) for a speed of the sensitive plate of 500 mm. per second. 

 We find then ??i rr= 9.4 X 10~^ ["^''^ — M-4]. 



Another series of measurements with the same string gave, at any 

 rate as far as the second figure, exactly the same result. The speed 

 of the sensitive plate was now 500 mm. per second. 



Besides c was =3.5 mm. per micrampere, 

 r ==1.16 mm. and 



k =1.83, 



from which we calculate again m= 9 A "^10^^ [mm — fi.4]. 



The agreement in the two final results is the more remarkable 

 because of the great difi'erence in the sensitiveness c and the period T. 



String 13 is thinner than 10, having besides a smaller conductive 

 resistance. The measurements with sudden passing of a constant 

 current, show that with F=r: 500 and c = 5.69, the value of T is 

 1.32 and that of k 3.1. From this we calculate from formula (7) 

 that the mass m is Q.dy<^10-^[mm — iiAj 



