in Magnetic Moment produced by Small Forces. 447 



I take the present opportunity of communicating some 

 observations and experiments upon each of these four mag- 

 nitudes. 



The results of the first part of the investigation, undertaken 

 conjointly with Mr. W. Hallock, will appear shortly in detail. 



I. On the Distance Apart of the Poles of Magnets. By W. 

 Hallock and F. Kohlrausch*. 



II. On the Increase and Decrease in Magnetic Moment produced 

 by Small Forces. 



We discuss here a single point, but a fundamental point in 

 the doctrine of induced magnetism. If we expose a steel 

 magnet to the action of feeble forces by which its magnetism 

 is either strengthened or weakened, the question arises whether 

 the increase and decrease in strength are of equal or of dif- 

 ferent magnitude. Lamontf has occupied himself with this 

 inquiry in his extended magnetic researches, and found that 

 the enfeeblement takes place more easily than the reinforce- 

 ment, and about in the proportion 4:3; and this view seems 

 to have been partially accepted by those % who have occupied 

 themselves with the subject. No thorough experimental in- 

 vestigation, however, appears to exist, and yet it is a subject 

 of the greatest importance in the more exact magnetic and 

 electrical measurements. Wild considers the determination 

 of the coefficient of induction as one of the chief difficulties 

 in the measurement of the intensity of terrestrial magnetism, 

 and, to avoid it, even proposes § to bring the oscillating mag- 

 net into a position at right angles to the meridian by means 

 of a bifilar suspension. 



Whilst it must be admitted that there is, in fact, a certain 

 difficulty in the determination of the two different coefficients, 

 yet this is much lessened if we have simply to determine their 

 sum according to the method of induction-currents given by 

 Weber || . If we have established a nearly constant ratio 

 between the coefficients, we of course need only to know 

 their sum. 



* A translation of this section was given among the short notices in 

 our last Number, p. 390. 



t Erdmaynetismns, p. 149 (1849). 



% Compare {e. </.)Wild, Rep. d. Meteor, d. K. Acad, zu Petersb. viii. No. 7, 

 p. 61 (1883), where it is mentioned that some observers, amongst theni the 

 author, have made no distinction between the coefficient of increase and 

 that of decrease. 



§ Wild, loc. cit. pp. 73 & 43. 



|j Abh. d. Gott. Ges. d. Wiss. vi. p. 65 of reprint, 1855. I cannot see 

 that the method suffers from complication, as H. Wild thinks. The accu- 

 racy required here at least is to be obtained with very simple apparatus. 



