34 Prof. H. F. Weber on Electromaqnetic and 



then the absolute value of one Siemens mercury unit (1 S. M. U) 

 in electromagnetic measure is : — 



G 2 M 1 7T 2 + \* ... 



1 S. M. U. = — s • STviTTn ' /— . • ( 4 ) 



n H 2T l( l + 0) M+X* 



Strictly, in the development of the absolute value for iv 

 account would also have to be taken of the fact that the cur- 

 rent induced primarily by the movement of the magnet is 

 variable with the time, and in consequence of this acts indu- 

 cingly on its own path. The carrying-out of the calculation 

 shows that the influence of this induction of the induced cur- 

 rent is so small in comparison with the other conditioning 

 moments, that the expression above given for w is, in conse- 

 quence thereof, only increased by (in round numbers) ^(Too* 

 Since, in the measurements cited below, none of the quantities 

 to be determined could be measured with such accuracy that 

 an additional - 2 ~o ooo °f ^s vame could have been safely esti- 

 mated, the influence of the induction on the part of the pri- 

 marily induced variable current might be completely ignored. 



For the determination of the absolute value of the S. M. (J. 

 by means of this procedure there are consequently seven dif- 

 ferent quantities to be measured. 



The five quantities \, \ 2 , T x , (1 + 0), and ( tt ) were deter- 

 mined by the method introduced by Gauss. The value of Gr 

 was calculated, by means of the fundamental law of electro- 

 magnetic action at a distance, from the dimensions and form 

 of the multiplier : 



^ r 1 _ 5 E 2 / R 2 \ | ^ /l _ 5 m 

 i+ R 2 \3 2p 2 \ p 2 )) p*\2 2p 2 / 

 2t™R 2 J 3 I 2 [-4D 2 - R 2 A 2 / 5 14 R 2 4D 2 -R 2 /21 . 21 D 2 \ ) 



G== -y-' < 4 P 2 L P * "vis" ^ + ~~7~ U + 2 y)i 



, V 2 f4 56 D 2 4D 2 -E 2 /7 21 D^\ \ "1 , 

 L + p 2 \3 3p 2 p 2 _ \6 2 p 2 ) J-!"*"-" 



Here n denotes the number of the turns of the multiplier, 

 R the mean radius of the turns, 2D the distance between 

 the central planes of the two spirals, 2A the height, and 2b 

 the breadth, of the cross section of the space occupied by 

 the turns, p the quantity \/R 2 + D 2 , and 21 the distance be- 

 tween the poles of the oscillating magnet. 



