Siemens Mercury Unit of Electrical Resistance. 297 



plier, since this was the process employed in the construction of 

 the English unit of resistance. 



II. On the Determination of the Absolute Unit of Resistance 

 canned out at the instance of the British Association*. 



The fourth method was proposed by Weber in two different 

 modifications : in one the axis of rotation of the inductor is 

 horizontal ; in the other it is vertical. In the former case it is 

 necessary to know the ratio of the two components of the earth's 

 magnetic force y inasmuch as the horizontal component acts on 

 the needle, while the inductive action is due to the vertical 

 component. By the second modification of the method, which 

 was the one adopted by the Electrical Standard Committee of 

 the British Association, this knowledge may be dispensed with, 

 since the horizontal component occurs, in consequence of its 

 inductive action, in the numerator of the expression for the 

 strength of the current, and as acting on the needle it enters 

 into the denominator ; so that we have here what is in truth a 

 most elegant, and, so far as the principle is concerned, a very 

 simple process, independent both of the earth's magnetism 

 and of that of the needle, and presenting only the mechanical 

 difficulty of the production of a constant and rapid motion of 

 rotation. 



But, on the other hand, another obstacle presents itself in 

 practically carrying out the second modification, the effect of 

 which comes out most clearly in the work of the " Committee," 

 and appears to call for more serious consideration than the 

 necessity of making a determination of dip. In this arrangement 

 the suspended needle, as well as the earth's magnetism, exerts an 

 inductive action; and consequently it is needful to eliminate the 

 effect of the former. If a small magnet, such as is commonly 

 used for galvanometric measurements, were to be employed in 

 conjunction with the multiplier of 300 millims. diameter adopted 

 by the Committee, its own inductive action would far exceed that 

 of the earth's magnetism. If the effect of the former was to be 

 treated as a small correction, it was accordingly essential to use 

 an unusually weak magnetic needle. 



The Committee went in reality very far in this direction — so 

 far, indeed, that such a weak magnetic needle as they employed 

 has assuredly never been used for any other measurement. Their 

 magnet consisted of a steel sphere (thus a mass of magnetically 

 the most unfavourable shape possible) of 8 millims. diameter and 



* Report of the British Association, 1862, p. 125 ; 1863, p. Ill ; 1864, 

 p. 345 ; 1865, p. 308. Where merely the number of the page is quoted 

 in what follows, the reference is to the Report for 1863. 



