Siemens Mercury Unit of Electrical Resistance. 347 



First determination. — (a) Temperature = 22°-5. The time of 

 oscillation of the needle-couple amounted to : — 



Without load 34-0771 and 34-0692. Mean . 34*0731 sec. 

 With load at the ends 56*8157 „ 



From this we have for the moment of inertia at 22°*5, 



56'8157 2 -34 2 0731 2 " 2016100000 = 1132400000; 



or at 17°, 11 32200000 = K'. 



(b) In order to eliminate the possible excentricity of the centre 

 of gravity of the weights, each of them was turned through 180°. 

 Temperature =17°. 



Without load 34-1486 and 34-1304. Mean . 34-1395 sec. 

 With load at the ends 56-9060 „ 



From which K" = 1133400000. 



Mean K, = i (K' + K") =1132800000 millim. 2 milligrm. 



Second determination. — Temperature =17°. The upper mag- 

 net was removed. It has the length 169'97, the semidiameter 

 6*95 millims., and the mass 199939 milligrms. Thus its mo- 

 ment of inertia amounts to 



199939 (i^V^) = 483800000. 



The time of vibration of the other part was : — 



Weights at the ends, 17-3717, 17'3720. Mean =17*3719 sec. 

 Weights in the middle 8*7154 „ 



Hence the moment of inertia of the whole is 



K *= 17-3719 7 ^8 2 7154^ 1996200000 ~ 195<)Q0QQ 



+ 483800000=1135700000 millim. 2 milligrm. 



Now the first mode of determination is open to the objection 

 that it has not as yet been determined whether the elasticity of 

 torsion of a wire is entirely independent of its load. In the 

 above case the elasticity formed the greater part of the directive 

 force. If, therefore, on this account we assign to the second de- 

 termination twofold value, the moment of inertia at 17° is 



K = 1134700000 millim. 2 milligrm. 

 Of this about £ would be due to the magnets, -J- to the brass 

 parts ; so that, for a temperature 0, 



K= 113 4700000 [1+0-000026. (0-17)]. 

 Time of vibration. — In the reduction to an infinitely small arc, 



