502 CONSTANTS OF COILS. 



If the support of the needle be turned through 180 about an 

 axis which passes through the middle of the frame, the mean of the 

 results obtained on either side corresponds to the semi-distance of 

 the two positions of the needle; but the experiment will then be 

 very complicated because the telescope and the scale must also be 

 moved. 



1090. It is better to arrange the experiment so as to get rid 

 of any measurement of the distance. The two coils, the surfaces 

 S and S' of which are to be compared, are arranged concentrically, 

 the axes coinciding. They are made to act on the same needle 

 placed at a great distance, and the deflections 8 and 8 l are observed 

 which are produced when a current passes through the coils in one 



direction or in opposite directions. Putting /3 = -- ^ , we have thus 



tan 6 



By taking the means, a series of alternate experiments enables us 

 to eliminate the variations of the principal current and the changes 

 of declination. If c and c', or rj or r/, are the terms of corrections 

 for the two coils in the two principal positions, 



i+c f 



or 



SI+ I+rj 



The distance of the needle from the centre of the coils only 

 comes then into the corrections, and may easily be determined with 

 sufficient accuracy. 



The method is particularly accurate when the surfaces to be 

 compared are near one another, the ratio /3 being then very small 



In order to eliminate the error arising from want of centring, 

 they are mounted on a frame which can turn about a vertical axis ; 

 and several series of experiments are made by turning through 180 

 the system of the two coils. 



In this case it is not even necessary that the coils have different 

 radii, so that one fits in the other. If the distance of the needle 

 is great enough, we may place them on either side of the axis. 

 The mean of the deflections observed for the positions gives very 



