492 Transactions. — Miscellaneous. 



On needle in A : First, third, fifth, seventh, &c, half-oscilla- 

 tions tend to magnetize needle in original direction ; second, 

 fourth, sixth, eighth, &c, tend to demagnetize needle. 



On needle in B : Second, fourth, sixth, &c, tend to mag- 

 netize needle ; first, third, fifth, &c, tend to demagnetize 

 needle. 



Now, the strength of field H in a solenoid of length large 

 compared with its radius is given by 



H = iirnc 



when n is number of turns per centimetre. 



Now, suppose that the solenoids A and B are of the same 

 number of turns per centimetre. Then the needle in B, since 

 it is acted on by the first half-oscillation, will be demagnetized 

 to a greater extent than the needle in A. The fall of the 

 deflection in every case was readily determined by the small 

 mirror magnetometer. 



Let the number of turns per centimetre on solenoid B be 

 reduced until there is exactly the same fall of deflection in 

 each needle after one discharge. The maximum magnetizing 

 force on needle in A = kirnc, where c is maximum current 

 of second half-oscillation ; the maximum magnetizing force 

 on needle in B = i-n-n'c', where n' = number of turns per centi- 

 metre, and c' = maximum current of first half-oscillation. 



Now, since the effects on the needles are identical in the 

 two cases, and the period is the same for both, the maximum 

 magnetizing forces in the two solenoids must have been equal. 



.'. iir-nc = A-n-u'c'. 

 c _ n' 

 •* c 7 — n ' 

 or the maximum currents of the two half-oscillations are to 

 one another inversely as the number of turns per centimetre 

 on solenoids. There is, of course, an assumption here that 

 the effect on the needles is : : al to maximum magnetizing force 

 when the period is constant. Experimentally it was found 

 that the depth of penetration of magnetic force was : : al to 

 the maximum current ordinate when period was constant, and 

 the assumption made is a very close approximation to the 

 truth. 



The connection between the depths of penetration when 

 the periods varied was more complicated, and not expressed 

 by any simple law. 



In experimenting it was found advantageous to pass about 

 twenty discharges instead of one, as the depth of penetration 

 was greatly increased, and also the action of the first effec- 

 tive oscillation was in a great measure differentiated from 

 the effect of the secondary ones. 



Many experiments on the relation between the amplitudes 



