296 M. F. Kohlrausch on the Absolute Value of the 



needle ; the magnetic intensity of the earth is not required, as its 

 effects compensate each other. It is, however, essential that the 

 needle should be short in comparison with the diameter of the 

 multiplier ; and consequently either the observations must be made 

 with a small needle, or the multiplier must be constructed of very 

 considerable dimensions. 



The second method*, which is the one employed in the fol- 

 lowing investigation, is a modification of the first, called for by 

 the very difficulties just referred to. A multiplier with narrow 

 coils and astatic needle serves as galvanometer, the dimensions 

 of which may be made to fulfil the conditions of greatest sensi- 

 tiveness, or those prescribed by any other considerations, in 

 whatever degree we please. This is possible, inasmuch as the 

 action of the unit current in the multiplier on the needle is not 

 calculated from the dimensions, but is ascertained empirically, 

 according to the laws of magneto-induction, by the damping 

 effect which the multiplier exerts on the swing of the needle. 

 But in addition it is now needful to know, not only the time 

 of vibration, but also the moment of inertia of the needle, as well 

 as the absolute value of that component of the earth's magnetic 

 force which acts on the inductor. 



The third method f is distinguished by great simplicity in the 

 necessary instrumental appliances, which consist simply of a 

 multiplier with a magnetic needle vibrating within it. If the 

 time of vibration of the needle is known, and if both the ratio 

 of the magnetic moment of the needle to the earth's magnetic 

 force and the distribution of magnetism in the needle have been 

 ascertained by observing its power of deflecting a compass, it is 

 possible from these data and the dimensions of the multiplier to 

 calculate the electromotive force developed in the latter by 

 the motion of the needle. The strength of the current thus 

 produced, and consequently the resistance of the multiplier, is 

 found by observations of its damping effect. 



The last process J consists in setting a multiplier of known 

 dimensions in rapid and uniform rotation, and observing the 

 deflection of a small magnetic needle suspended within it. 



In each of these methods, whenever great accuracy is aimed 

 at, we encounter difficulties of execution, by no means inconside- 

 rable, arising from sources to which attention is called. by italics 

 in the foregoing paragraphs. We must next consider somewhat 

 more in detail the last-mentioned process of the rotating multi- 



* Abhandl. d. K. Gesellsch. d. Wissensch. zu Gottingen, 1862, vol. x. 

 p. 20. Also printed separately under the title " Zur Galvanometrie" 

 Gottingen, 1862. 



t Abh. d. K. Sachs. Ges. d. Wiss. 1846, vol. i. p. 232. 



% Weber, Zur Galvanometrie, p. 12. 



