162 



A dams — Electromagnetic Effects of 



the force at the lower needle obtained by time-integration of 

 the curve be : 



2^q . 



and the mean value of the force at the upper needle : 



2ttN^ 



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Then the effect on the needle will be the same as if constant 

 forces of these magnitudes acted upon it. The same result 

 could be obtained by imagining two coils of wire passing 

 through the centers of the two sets of spheres through which 

 a current was sent in opposite directions of such a magnitude 

 that the same amount of electricity passed any point per 

 second as in the case of the charged spheres. 



