158 Dr. W. F. G. Swann on the Magnetic Field produced 



reversed through each coil, and the experiments were 

 repeated. A measurement of the time of swing of the 

 galvanometer completed the set of observations. 



Magnitude of the effect to be expected due to the 

 Eartlis Motion. 



The expression for the maximum flux through the coil, 

 given on page 151, becomes 



i x 10 ~ l2 v (/,— !) j V%, 



where k and 1 are the specific inductive capacities of paraffin 

 wax and air respectively, and V is in volts. 



If I is the length of the line formed by the intersection of 

 the plane of the coil with the dielectric, and if V : and V 2 

 are the respective mean values of the potentials over the 

 corresponding lines for the two coils, we have 



Maximum flux through the two coils 



= B = J x I0~ 12 vl (/„— 1) (Vi + V 2 ). 



The method of measuring Y 1 and V 2 is explained on page 162. 



In order to determine the magnitude of the deflexion 

 which the above flux should produce when the coils rotate, 

 it was best to short-circuit one of the coils (which was done 

 by means of a brass collar connecting the copper strips 

 joining the coils) , set the other coil in rotation in such a 

 position as to be affected only by the earth's vertical com- 

 ponent, and observe the deflexion B 2 produced on the galva- 

 nometer. A high resistance was put in series with the 

 galvanometer, and the pair were shunted for this purpose. 



Let R 2 be the resistance of the circuit containing the single 

 unshort-circuited coil and the shunted system, G, Z, and S 

 the resistances of the galvanometer, the coil in series with it, 

 and the shunt respectively ; let a be the effective mean area 

 of the coil, n 2 the frequency of rotation of the coil, and W 

 the vertical componentiof the earth's field; then the required 

 deflexion which would be produced by the flux B is easily 

 seen to be 



where Ri is the resistance of the two rotating coils and the 

 galvanometer in series, % is the frequency of rotation in 

 the main experiment, 1\ and T 2 are the times of swing of 



