H. V. Gill — Electric Discharge in Geissler Tubes. M)l 



production of longitudinal wave disturbances, of the same 

 nature exactly as those studied in acoustics. These disturb: 

 nave been investigated by several physicists, but with perhaps 

 the greatest success by E. Mach * We give briefly the resnua 

 of some of his experiments. 



When two sparks were made to take place simultaneously, he 

 proved the laws of interference were fulfilled exactly as in tin- 

 case of air waves studied in acoustics. The velocity of the 

 wave is at first greater than that of a sound wave, but at some 

 distance from the point of discharge becomes equal to it.f In 

 order to study these interferences he employed a plate of 

 smoked glass, and the sparks took place over its surface. The 

 curve of interference was registered by lines on the glass where 

 the black covering was removed by the waves at their points 

 of interference. For example, one spark took place along a 

 metalized string parallel to the surface of the glass and quite 

 near to it, thus producing a line of sparks between the particles 

 of metal on the string: a second spark passed from a terminal 

 vertically over the plate, forming a spark at a single point. The 

 waves due to the first spark evidently formed a cylindrical 

 surface with the string as axis, while those due to the second 

 formed a spherical surface around the point. The line of inter- 

 ference on the surface of the glass was, as theory demands, a 

 parabola having the point for focus and the line as directrix. 

 The surface of interference could be determined by moving 

 the plate of glass into various positions in space. A wire con- 

 taining two breaks was placed between two plates ; the direct 

 waves interfered with the reflected ones. 



In all these cases the lines and surfaces of interference obeyed 

 the laws of longitudinal waves in an elastic medium. 



The velocity of the wave was found to increase with the 

 intensity of the discharge. We call special attention to the 

 following remark. He determined by an ingenious method 

 the fraction expressing the density of compression to be 

 <7=0-15. "The experiments of Toepler and Boltzmann give 

 <7= 0,0035 for the condensation of a column of air in vibra- 

 tion near a node (in an organ pipe); we see therefore that the 

 condensation in the air waves produced by even feeble Bparka 

 is much greater than that observed in sounding pipes." 



It follows from these results that if the oscillations of a 

 spark could be rendered sufficiently slow, a musical note ought 

 to be obtained. Dr. Lodged has actually obtained this result, 

 and reduced the frequency of oscillation to 500 per second. 



* Vienna Academy, May-July, 1878; Journal de Physique 1 1 ). viii. 



f It seems more probable that this falling off of the velocity is more apparent 

 than real- we are inclined to think that the -renter velocity near the B] 

 due to local effects such as the electrical state of the air in its vicinity, the move- 

 ment of the air near the spark, etc. 



% Modern Views, pp. 376-377. 



