450 . Messrs. Eowland and Hutchinson on the 



a " series '^ of readings consistino- of about 25 reversals Avas 

 made. After the series, electrometer readings were again, 

 taken; the conditions were then changed in some way, and 

 another series begun. 



The circumstances to be changed are : — distance of disks 

 from needle ; distance of glass plates from needle ; electrifi- 

 cation ; and direction of rotation. 



The calculation of the deflexion is based on the assumption 

 that the magnetic effect of a rotating charge is proportional 

 to the quantity of electricity passing any point per seconci, 

 just as with a conduction-current. Below are the formulae 

 used. 



In the equations the letters have the following meanings. 

 All quantities are given in terms of C.G.S. units. 



X = Distance from centre of disk to lower needle. 



7* = „ „ „ upper needle. 



c = Radius of disk. 



/ = Distance between needles. 



a = Radius of windings of disk-galvanometer. 



h = Distance, centi-e 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. 



^ = Angle of deflexion of sine-galvanometor. 



h = Angle of deflexion of tangent-galvanometer. 

 A = Change of zero-point on electrifying the disks = half 

 the charge on reversing. 



s = Scale-reading for disk-galvanometer. 

 to = Weight on pan of electrometer. 

 D = Electrometer reading, 



e = Distance of glass plates and disks 

 = Condenser distance. 



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

 ' radius c, at a distance .v on the axis 



= 27rl 7"^-- ^3. 

 Strength of convection-current 



= 27rcdc-^. 



