CHAPTER X. 

 ELECTEIC TRANSMISSION IN VACUA. 



ELECTRIC TRANSMISSION IN Low VACUA. Let a 

 glass tube, about thirty inches long, be provided 

 with brass caps at each end, fitting air tight; from 

 each of which a pointed brass rod projects inwards. 

 And let a stop-cock be attached to one of the caps, by 

 which the tube can be connected with an air pump, as 

 shown in Fig. 43. 



Let the tube be insulated, and the caps connected by 

 conducting cords with the balls surmounting the Ley- 

 den jars of the Topler machine; the sliding electrodes 

 being separated to their full extent. When filled with 

 air, at the ordinary atmospheric density, it will be found 

 impossible to pass an electric charge through a tube of 

 this length: but let it be connected with an air pump, 

 and the air well exhausted, and a charge Avill easily pass 

 through. This proves that air at the ordinary density 

 has a much higher electric resistance than rarefied air. 



But if a high degree of vacuum is produced, it will be 

 found much more difficult to pass the charge through ; 

 which indicates that a medium, consisting of some 

 material substance, is essential to electric existence 

 and movement; and that if it were possible to produce 

 an absolute vacuum, electricity could not pass through. 



If the above experiment be performed in a dark room, 

 flashes of red and violet colored light will be seen to 



