60 



Prof. J. C. Bose on a Complete Apparatus for 



a potentiometer-slide arrangement. This is a matter of great 

 importance, as 1 often found a receiver, otherwise in good 

 condition, failing to respond when the E.M.F. varied slightly 

 from the proper value. The receiver, when subjected to 

 radiation, undergoes exhaustion. The sensibility can, how- 

 ever, be maintained fairly uniform by slightly varying the 

 E.M.F. to keep pace with the fatigue produced. 



The receiving circuit thus consists of a spiral-spring coherer, 

 in series with a voltaic cell and a dead-beat galvanometer. 

 The receiver is made by cutting a narrow groove in a rect- 

 angular piece of ebonite, and filling the groove with bits of 

 coiled steel springs arranged side by side in a single layer. 

 The spirals are prevented from falling by a glass slide in 

 front. The spirals are placed between two pieces of brass, of 

 which the upper one is sliding and the lower one fixed. 



Fig. 3.— The Spiral Spring Eeceiver. 



These two pieces are in connexion with two projecting metallic 

 rods, which serve as electrodes. An electric current enters 

 along the breadth of the top spiral and leaves by the lowest 

 spiral, having to traverse the intermediate spirals along the 

 numerous points of contact. The resistance of the receiving 

 circuit is thus almost entirely concentrated at the sensitive 

 contact-surface, there being little useless short-circuiting by 

 the mass of the conducting layer. When electric radiation is 

 absorbed by the sensitive surface, there is a sudden dimi- 

 nution of the resistance and the galvanometer spot is violently 

 deflected. 



By means of a very fine screw the upper sliding-piece can 

 be gently pushed in or out. In this way the spirals may be 

 very gradually compressed, and the resistance of the receiver 

 diminished. The galvanometer spot can thus easily be 

 brought to any convenient position on the scale. When 



