METHOD OF DAMPING. 541 



It is necessary therefore to follow the variations of the horizontal 

 component during the experiments. 



1121. It is worthy of notice that experiments made by the 

 method of damping, have always given higher numbers for the 

 resistances, and therefore smaller values for the unit. 



The calculation assumes that the deflections are very small, while 

 we are led by experiment to observe considerable deflections, so as 

 to facilitate the measurement of the damping, which should be rapid. 

 In these conditions we cannot assume that the constant G is inde- 

 pendent of the elongation. Approximations made by calculation on 

 the hypothesis of small deflections do not seem to furnish a sufficient 

 explanation of the disagreement between this method and the others. 



The magnetisation of the bar by induced currents themselves 

 (885) plays a part which it seems cannot be neglected.* These 

 currents are very strong when the current is considerable, and they 

 impart to the bar a transverse temporary magnetisation. 



When the bar oscillates on either side of the meridian, the tem- 

 porary intensity of magnetisation of the bar is proportional to the 

 action GI of the current, and may be represented by /GI. If V be 

 the volume of the magnet, the corresponding moment is /GI V. This 

 magnetisation scarcely modifies equation (12)' of 845 relative to the 

 induction, for the work of the current on the transverse magnet is 

 almost null ; but the action of the earth brings into equation (14)' a 

 couple H/GIV of the opposite sign to the couple MGI, so that the 

 second member of this equation should be replaced by 



MGI - H/GIV = mGI 



( i - 



it follows, finally, that the second member of the value of R should 



H/V 

 be multiplied by the factor i . 



Now, if I a is the mean intensity of the principal magnetisation 

 of the magnet, I t that of the earth, and if we suppose the experiment 

 made in the magnetic latitude of 45, we have (153) 



H = - irl t cos 45 = 7rl t ; 



* MASCART. Comptes rendus, Vol. c., p. 313. 1885. 



