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the Poles of the Electric, Arc. 767 



component, H P . When B is +, the current direction is 

 OMB, thus according with Fleming's law, the repulsion 

 is apparently increased. 



The resultant H„ . OM . i of the forces acting on OM is 

 applied to the centre of OM, hence on transferring it to the 

 end M it is to be reduced to 



OM 

 H„.-2-.t. 



In a similar way, the couple due to the forces acting on 



"Diy/r 



BM being (H p . BM . i) .— s— » we may replace it with a forca 

 (H,..BM.0 2 ™ m 



applied to the extremity of the arm. As this force is much 

 less than the preceding, when the length of the carbon rod 

 BM during a set of experiments is reduced by burning, no 

 appreciable error is caused. 



Adding the two forces together, we have 



OM BM 2 . 



V = Hr -"T l + H, '20M t ' 



or, by setting the current I in amp., 



H^OB* 

 20 OM W 



This simple calculation of V may bring a remarkable 

 economy in carrying out further experiments. Of course, it 

 would be better to compensate both the actions V and E by 

 some magnet or circuit conveniently disposed. 



In my apparatus OM = 15 cm., MB = — 4 cm., H^ = 037 

 (Palermo), then 



V= 0-296 1 . (1') 



Much more laborious and doubtful would it be to calcu- 

 late E, i.e. the coefficient K of "the law 



E = KP, (2) 



whenever the geometric data of the circuit were known. 

 It is then preferable to estimate it experimentally, as Prof. 

 Duffield did. He found (p. 124) with 8 amp. E = 75 degrees 

 = 1*8 dyne, thus 



K = 0'028, 



E = 0-03F (2') 



