ACOUSTICS AND GRAVITATION. 13 



respectively receives a positive charge while the other electrode and the mer- 

 cury is earthed. This affords a satisfactory means of commutation; for since 

 AV=C\/n = C'\/ r n' if the electrodes are so adjusted that C and C' are 

 nearly equal, 



A V = 



Tests were made with this apparatus and a known AF=i73 volts. For 

 example, the scale-readings in the ocular on commutation were # = 34, #' = 17, 

 so that (x x')=o.7 (w+n'), as the fringe breadth was 0.7 scale-parts. Thus 

 AV = A\/24^ or A =35. 3 volts per fringe, initially. With large fringes and 

 under quiet surroundings 3 or 4 volts could have been detected. 



To endeavor to carry the sensitiveness beyond this, either by the outside 

 mercury receiver or by cautious manual adjustment of C and C', did not suc- 

 ceed. This would have to be done by special mechanism. 



Similarly the experiments made to put in a guard-ring led to no results of 

 consequence, as the sensitiveness decreases rapidly if only the central parts of 

 the mercury surfaces are utilized. Conversely, the central perforation of 

 the electrodes may be enlarged to nearly 2 cm. in case of electrodes nearly 

 fitting the air-space, without appreciable fall of sensitiveness. If wire gauze 

 is placed over these holes the interferences are not destroyed (though they 

 become weaker), but the electrical advantage is negligible. 



