250 



DISCHARGE OF ELECTRICITY. 



produce a <i,low dischiirge from the glass into the bulb, and may thus 

 furnish a supply of dissociated molecules through which the ordinary ring- 

 discharge can pass with much greater readiness. For nothing is more 

 striking than the enormous difference produced in the electric strength 

 of these rarefied gases by the passage of a spark. It is sometimes 

 difficult to get the discharge to pass at first, but when once a spark has 

 passed through the gas, a spark length one- quarter the length of that 

 necessary to originate the discharge will be found sufficient to main- 

 tain it. 



It is sometimes convenient, in cases where difficulty is found in start- 

 ing the discharge, to avail ourselves of this property by connecting the 

 mercury of the pump to which the tube is attached with one terminal 

 of an induction coil, the other terminal of which is i^ut to earth. When 

 tlie induction coil is in action, a glow-discharge fills the pump and tube, 

 and while this glow exists the electrodeless discharge can easily be 

 started; once liaving been started, it will continue after the induction- 

 coil is stopped. An experiment of this kind, which I had occasion to 

 make, gave very clear evidence of the way in which dissociated mole- 

 cailes are projected in all directions from the negative electrode in an 

 ordinary discharge tube, but not from the positive. The discharge tube 

 was fused onto the pump, and at an elbow two terminals, c and d, Fig. 

 II, were fused into the glass; these terminals were connected with an 



induction coil, and the pressure in the tube 

 was such that the electrodeless discharge 

 would not start of itself. When the coil was 

 turned on so that c was the negative electrode 

 the electrodeless discharge in the tube at 

 oace took place, but no effect at all was pro- 

 duced when (' was jjositive and d negative. 

 We may thus regard the effect produced by 

 the i)resence of a conductor as due to the 

 conductor catching the tubes of electrostatic; 

 induction and concentrating them on the 

 O discharge tubes; these tubes in many cases 

 acting indirectly by producing a glow dis- 

 charge through the tube, which, by diminish- 

 ing the electric strength of the gas, makes 

 discliargesof any other kind very much easier. 

 Fig. 14. Though the presence of a conductor near the 



discharge tube will, in general, concentrate the tubes of electrostatic 

 induction on the discharge tube more than would otherwise be the case, 

 yet this does not always happen. When in some positions the conduc- 

 tor may hold back for a time from the discharge tube tubes of electro- 

 static induction which would otherwise pass through it, and thus 

 diminish tlie maxinuim density of the tubes of electrostatic induction 

 in the discharge tube, and hence tend to stop the discharge. I have 



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