SHUDDEMAGEX. — DEMAGNETIZING FACTORS FOR IRON RODS. 231 



R = total resistance of the circuit. 



If we apply this equation to our secondary coil circuit, which includes 

 the ballistic galvanometer, we have, since E=0, 



Jo 



or Q = \X/R, 



where Q = total charge through galvanometer, 



^JSf = number of flux-turns of change in the magnetic induction 

 through the circuit. 



This equation is expressed in c.g.s. units. If we use as our units 

 the ampere, ohm, microcoulomb, and gauss, as we have done, then we 

 must use the equation, 



Q = Ai\7(100i?). 



We have also Q = T/S, 



where T = actual throw in centimeters of scale reading produced by 

 the discharge of Q microcoulombs through the galvanometer, and *S' = 

 sensitiveness of galvanometer, expressed in centimeters of deflection 

 obtained by discharging 1 microcoulomb through the galvanometer. 

 Now in the reversal method as used in these experiments, 



Ai\" = 2 BA}i = 2 B7r(D/2)\ 



wiiere B = the magnetic induction in gausses, or number of lines of 

 induction per square centimeter passing through the middle of the iron 

 rod, 



A = cross-section of rod in square centimeters, 



n = number of turns of secondary coil ^ound around the middle of 

 the rod, 



D = diameter of the rod, as before. 



This gives us 



2 Biv{D/'2)-n _ T 



100 • R ~ S ' 

 5 100-^ 



or 



T 2 STT{D/2f ■ n 



This formula is the most convenient for our purposes. As in our series 

 we had the data 



