PAPERS ON PHYSICS 307 



extension of the discharge tube, and which carries over a 

 considerable portion of its length a secondary" of many 

 turns of copper wire. The cathode ray solenoid, i. e., the 

 spiral cathode ray beam within, is thus made the primary 

 of a vacuum cored transformer, the secondary of which is 

 connected to a low resistance high sensitivity galvanometer. 

 The charge caused to circulate through the galvanometer 

 on interrupting the cathode ray beam, or on allowing it to 

 pass, is, to a first approximation, 



Q= K.d = 471 ^ij^ . ■ . . (1) 



where n^ is the number of turns per centimeter length of 

 the primary, n^ the total turns in the secondary, A the 

 mean area of the primary-, I the current in amperes flowing 

 through the cathode ray solenoid, and R the total resistance 

 in the secondary. Since the secondary has a comparatively 

 low resistance the damping of the galvanometer will be 

 considerable, and hence its ballistic constant, K, becomes a 

 function of the resistance R. Under these conditions the 

 galvanometer may be calibrated readily and thus the order 

 of the deflection that should result on making or on inter- 

 rupting the beam, for a given current I, becomes known. 



As in experiments of this kind, it is wise to test the set- 

 up before going farther and get some idea of the magnitude 

 of the quantity of electricity induced in the secondarj^ by 

 known values of I made or broken in the primary, i. e., to 

 see whether the effect sought will produce measurable de- 

 flections of the galvanometer. To make this preliminary 

 test we placed down through the central tube, where the 

 cathode ray spiral was supposed to go, a copper wire spiral 

 having the same pitch (turns per centimeter) and diameter 

 as the proposed solenoidal cathode ray spiral. Then, on 

 making or breaking the current through this, we read the 

 corresponding induced current in the secondarj^ — this being 

 proportional to the deflection of the galvanometer. Thus 

 deflections of the galvanometer were obtained for a number 

 of different values of I in the primary. The values are con- 

 tained in Table I. 



