256 HEXEY A. ROWLAND 



In the equations the letters have the following meanings. All quan- 

 tities are given in terms of C. G. S. units. 



X= Distance from centre of disk to lower needle. 

 r = Distance from centre of disk to upper needle. 

 c = Radius of disk. 

 I = Distance between needles. 

 a = Radius of windings of disk-galvanometer. 

 i = Distance, centre of disk-galvanometer to lower needle. 

 p = Distance, centre of disk-galvanometer to upper needle. 

 N = Number of revolutions per second. 



a = Surface-density of electrification in electrostatic measure. 

 V= Ratio of the units. 



a = Angle of torsion of the electro-dynamometer. 

 <f> = Angle of deflexion of sine-galvanometer. 

 8 = Angle of deflexion of tangent-galvanometer. 

 J = Change of zero-point on electrifying the disks = half the charge 



on reversing. 



* = Scale-reading for disk-galvanometer. 

 w = Weight on pan of electrometer. 

 D = Distance of glass plates and disks. 

 ^ = Electrometer reading, 

 z = Condenser distance. 



Force, in the direction of the axis, due to a circular current of radius 

 c, at a distance x on the axis 



Strength of convection-current 



NT 



.'. total force due to the disk of radius c 



_ 4 ^ _ _- 



~ ~V 



and for the two disks acting in the same direction, total force 



T_Q_2 Na A 

 V A ' 



This gives the force on the lower needle. 



