436 Specific Resistance of Mercury. [Nov. 20 y 



Then M (n^ w 2 ) = -r p, 



where M = the coefficient of mutual induction of coil and 



disc ; 



p = the specific resistance of mercury ; 

 A = area of section of the mercury column. 



The capillary depression at the sides of the trough would make it 

 a serious task to determine the section of the mercury column 

 by direct measurement to the required degree of accuracy. This 

 difficulty is overcome hy a further differential method, viz., by making 

 observations with the mercury at two different heights in the trough. 



Let b = the breadth of the trough ; 

 ~h z li-^ = the difference of height of the mercury surface in the 



two cases ; and let 



A = the section of the mercury column when the mercury is 

 at the lower position. 



Then we have, denoting by dashed letters the new values of the 

 rates of rotation and the distance between the corresponding equi- 

 librium positions 



and . M < 



whence, eliminating A, 



f> ~ 



M (W T ,) = 

 ' 



It is assumed in the above formula that the sides of the trough in 

 that part of it traversed by the movable electrode are plane, parallel, 

 and vertical. 



The trough used in the experiments described was cut in paraffin 

 wax, contained in a strong casting of iron, with its sides strengthened 

 by outside ribs. The channel is approximately 43'5 inches long by 

 1'5 inches broad by 3 inches deep. It was first cut by a cutter 

 rotating about 2000 times a minute, attached to the slide rest of the 

 College Whitworth lathe, and subsequently finished by a scraper, 

 attached in similar fashion, which took a very thin cut off sides and 

 bottom. The result of the scraping was a very smooth and highly- 

 finished surface. 



