Effect of Electrical Convection, 45 



the induced coil a distance equal to the thickness o£ the disk, 

 and the resultant change A in the galvanometer-deflexion 

 noted . The frame was then pushed up into its former position, 

 and the change in deflexion again noted. A second known 

 current i 1 was then sent through the reverser and the test- 

 coil on the frame carrying the disk, and the change in galvano- 

 meter-deflexion B resulting from a known change (i 1 — i 2 ) in 

 this current noted, i, i 1; and i 2 were so chosen that the 

 deflexion produced by the current i x in the test-coil was equal 

 to the deflexion produced by the current i in the coil on the 

 surface of the disk, and the change in deflexion A was approxi- 

 mately equal to the change in deflexion B. In this way the 

 quantities A and B were measured at the same part of the 

 galvanometer-scale, thus avoiding any error due to a lack of 

 proportion between the current and the deflexion, which was 

 considerable in the galvanometer employed. Let p 1 be the ratio 

 of the deflexion produced by unit current flowing through the 

 reverser and any coil on the surface of the disk next to 

 the induced coil to the deflexion produced by unit current 

 flowing through the test-coil, p 2 the corresponding quantity 

 when the coil is on the opposite surface of the disk. Then 



i B 



From the observations taken as above described p\ — p 2 was 

 calculated and plotted for twelve different coils. 



Let A be the deflexion produced by a unit current flowing 

 through the reverser and test-coil. Then 



S-S'=iA( Pl -p 2 ); 



(8 — & is the deflexion due to the making and breaking of a 

 current, whereas A and B are the deflexions resulting from 

 a reversal of current, hence the factor -J). The formula for 

 D therefore becomes 



D=- KPi—P2) dr - 



Olll 



The integral I r(pi — p 2 )dr was calculated graphically ti- 

 the plat of pi~p 2 . 



A number of observations of the deflexion D were made. 

 which always agreed in direction and fairly well in amount 

 with the deflexion as calculated. A close agreement could 

 not be expected, inasmuch as the assumption that a\< uniform 

 at the edge of the disk is only a rough approximation to the 



