Soft Iron, especially with weaker decomposing-powers* 51 



In every observation, the temperature of the air close to the 

 multiplier was noted; and a second thermometer was applied to 

 the second layer of the ring, which with more powerful primary 

 currents was considerably heated by contact with the primary 

 wire. The resistances were reduced to the readings of the two 

 thermometers ; and herein the increase of a resistance w (of cop- 

 per) with a heating of 1°C. could be supposed equal to OG0387w*. 



The number above denoted by m } which specifies in what ratio 

 the action of the multiplier was more powerful than that of the 

 roll (the currents in both being equal), was measured once for all. 

 For this purpose the current from a galvanic apparatus was con- 

 ducted through the roll; a known portion of the same current 

 was sent through the multiplier by means of a derivation of 

 small, accurately measured resistance. There was found m — 

 2414 for the case in which the roll was 1000 millims. distant 

 from the suspending thread of the magnet. From the constants 

 of the roll to be given below, the corresponding number m could 

 be calculated for the case in which the distance was different. 



We have now all that is necessary for the calculation of k 

 from the observed A, a, X, and T. In order, however, that we 

 may be able to calculate also the argument R (the magnetizing- 

 force) to which this k refers, the intensity of the current i must 

 be determined in absolute measure. Let 



H be the horizontal direction-force of the magnet (proceeding 

 from the magnetic field of the place of observation, in a 

 small part also from the. torsion of the suspension-thread), 



u the angle of deflection of the magnet, produced by a current 

 of absolute value i which is passing through the roll, 



F the superficial extent of the roll, 



r the distance of the roll from the thread of the magnet ; 



then we have, as a first approximation :— 



B.tanu _2¥ . 



i " r 3 



More exactly, the right-hand part of this equation is a series 

 which proceeds by descending powers of r. Presupposing that 

 both magnet and roll, in their distant magnetic action, are sym- 

 metrical in relation to their axes, only odd powers of r can occur 

 in that series. Limiting ourselves to the first two terms, we 

 obtain 



H tan u __ 2F / (3\ 



where J3 is a constant f which depends on the dimensions of the 



* Matthiessen and v.Bose ; see Wiedemann, Galvanismits s vol. ii. p. 1060. 

 + The influence of the angle u on /3 may be neglected when u is small. 



E2 



