134 



DR. HAROLD A. WILSON ON THE ELECTRIC EFFECT OF 



as shown to a brass tube TT, through which the wires leading to the coils were 

 brought out. The coil C consisted of a single layer of fine silk-covered wire having 

 9 turns, and its mean diameter was 4 '28 centhns. The ends of the wire were 

 twisted together and connected to an Ayrton-Mather ballistic galvanometer. The 

 coil C' was similar to C, and its mean diameter was 2'01 centims. The deflection of 

 the galvanometer coil, due to reversing a known current in the solenoid, was deter- 

 mined for each of the coils C and C' at a series of positions along the axis of the 

 solenoid. The coils C and C' were then put at the centre of a solenoid, 50 centims. 

 long and 5 '4 centims. in diameter, consisting of a single layer of wire, with 4 '37 5 

 turns per centimetre, wound on a brass tube. Known currents were then reversed 

 in this long solenoid and the galvanometer deflections with the coils 1 C and C' 

 measured. The current was passed along the brass tube to neutralize the field due 

 to the component of the current along the solenoid. The field strength at the centre 

 of this solenoid was calculated, and so the sensibility of the galvanometer with each 

 of the coils C and C' was obtained. The magnetic field due to the solenoid used to 

 produce the field in which the cylinder was rotated was found to be proportional to 

 the current through it throughout the region occupied by the cylinder. The following 

 table gives the results obtained for the change in the mean field strength due to 

 reversing one ampere for each of the coils C and C' : 



The mean field through the coil C, due to a current of 2 amperes, over the length 

 of the cylinder calculated from these numbers is 200 '0, and that through the coil C' 

 is 199 "4. The mean induction through the ebonite is consequently 



200 



199 '4 irr 2 = 20G7. 



