52 Prof. E.Wiedemann on the 



large production of heat, and that this decreases, reaching a 

 minimum within the dark kathode-space, then it rises again 

 to a maximum within the glow-rays, and sinks again towards 

 the positive light. 



It is therefore, according to our previous experiments, not 

 possible to deduce the change in heating-effect from the 

 change in resistance. We have not the maximum heating 

 exactly at those places where the greatest resistance is offered 

 to the radiation of the positive light. There are now two 

 things possible, either (1) p is not zero — that is, free electricity 

 moves within the tube ; or (2) apart of the fall in potential is 

 used up at those points where the heating-effect is a minimum 

 in imparting to the electricity a certain kinetic energy, which 

 disappears again at the points of maximum heating-effect. 



I shall endeavour to determine which of these two views is 

 the correct one by special measurements of potential within 

 the tube. 



4. Deflection of the Positive Column of Light. — Two circum- 

 stances are of importance in connexion with the deflection of 

 the sensitive positive light, which takes place upon touching 

 the tube traversed by the discharge with the finger ; it some- 

 times consists in an irregular attraction, sometimes in a 

 symmetrical deflection. 



The one circumstance is the change in the distribution of 

 free electricity upon the wall of the glass tube, by which the 

 potential is lowered at the point touched, and the discharge is 

 deflected towards the point touched ; this is especially the 

 case at high pressures. The second circumstance is the 

 formation of a dark space at the point where positive elec- 

 tricity flows into a conductor, or becomes bound. The ex- 

 periments with the floating electrode show, indeed, that the 

 passage of the positive electricity through the dark space 

 meets w T ith great resistance, and then the discharge bends 

 round backwards. But so soon as any point of the wall is 

 put into connexion with the ground, a kathode is produced at 

 that point, kathode-rays issue from it, and the peculiar 

 reddish light is seen ; consequently a dark space must form 

 around this point, which prevents a portion of the tube from 

 being traversed by the positive light, round which the dis- 

 charge therefore bends. 



Exactly similar reasons explain the luminosity of the tube 

 when, the discharge being such that the tube remains dark, it 

 is encircled by the fingers, or by a ring of tin-foil. Then the 

 whole wall at the points touched becomes a kathode, and 

 opposes to the discharge a great resistance, which is, of 

 course, attended by a loss of energy, and has for consequence 



