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Mr. A. Schuster. 



Some Questions relating to the Positive Discharge. 



There is one theory of electric discharges which, as a scientific 

 curiosity, is of interest. It asserts that a perfect vacuum is a perfect 

 conductor, but that the molecules flying about the vacuum, impede 

 the passage of the current. If no discharge actually passes through 

 highly rarefied gas in an experiment, it is, according to this view, 

 because there is a resistance at the surface of the electrodes. It is 

 interesting to speculate what the world would be like if this theory 

 was true. It is perhaps not fair to urge that we should live in per- 

 petual darkness, because the upholders of the theory could not con- 

 sistently adopt the electro-magnetic theory of light, whose essence is a 

 stress in the medium. But I do not see how we should have any elec- 

 trostatic effects at all, at any rate in highly exhausted vessels. In order 

 that gold leaves should remain divergent in vacuo, it is by no means ne- 

 cessary that no escape of electricity should take place from them. They 

 will collapse, though their surface may be perfectly impermeable to the 

 discharge, as long as currents may flow in the surrounding medium. If 

 the gold leaves are charged positively, negative electricity would flow 

 towards them, cover them, and protect the leaves, so as to prevent 

 any repulsion. There are other fatal objections against the theory 

 which will survive nevertheless, for, like all paradoxes, it has an 

 irresistible attraction to a great many minds. 



The following experiment has convinced those to whom I have 

 been able to show it that the discharge consists of a diffusion of 

 charged atoms or molecules. The apparatus used is that by means 

 of which Balfour Stewart and Tait have carried on their researches 

 on the heating of a rotating disc in vacuo. In front of an ebonite 

 disc, electrodes were introduced so that the line joining them was 

 parallel to the plane of the disc, viz., one electrode opposite the 

 centre, and the other opposite the edge of the disc. When the latter 

 is rotated, it carries round with it the air in its neighbourhood. If 

 the current consisted of a motion of the medium, the particles of air 

 could not affect the distribution of the lines of flow. But it is found 

 that the discharge is drawn up or down according as the motion of 

 the air is upwards or downwards. The curves formed by the dis- 

 charge are similar to those observed when a magnet acts on a 

 positive discharge, and their cause is identical, as the magnetic force 

 also acts on the particles, and tends to draw them through the 

 discharge, as I have explained in my previous Bakerian Lecture. 



Photographs of the actual appearance have been taken, and will 

 be given in the full account of these experiments. It is very striking 

 to see the discharge steadily and slowly deflected by the rotating disc, 

 and so sensitive is it to slight currents in the air, that the heating 

 of the gas by the discharge and the convection currents formed in 



