464: PROCEEDINGS OF THE AMERICAN ACADEMY 



The magnetic induction = total number of lines of force divided by 

 the area of the section ; therefore 



B = ^^^ M — H, M. 

 a 



» 



In order to obtain the magnetic induction in absolute measure, the 

 deflections of the helix were compared with the deflection of an earth 

 inductor in the same circuit, thus : — 



or 



„ N'lrRWh 



Similarly, 



"We have seen that 



Tvr^NC 

 2N'T vRWh' 



2N^y 2N^K tancr. s 



a a 



The reduction factors of the tangent galvanometer used in Tables 

 I. and II. are : 



The reduction factors of the tangent galvanometer used in the other 



tables are 



K, = .008868. 



iT, = .01972. 

 K^ = .0876. 

 A\ = .4435. 



Both the tangent and the mirror galvanometer were placed upon the 

 stone piers of the laboratory. 



C is the mean of from six to ten readinss ; h and h' are the mean 

 of four ; and 5 is the mean of four readings, two before and two after 

 // and h' were taken. 



The vertical component of the earth's magnetism at Cambridge, 

 Mass., is 0.5406. 



