194 Mr. A. Schuster on the Disruptive Discharge 



different from that found for two spheres placed at such a 

 distance that the disruption-stress is a minimum. Without 

 attaching too great an importance to this apparent coincidence, 

 I think we may safely conclude that the explosive distance is 

 really a function of the curvature of the surface. 



The second result which may be deduced from Bailie and 

 Paschen's observations is, in my opinion, also well established. 

 The fact that the breaking-stress diminishes for increasing 

 distances between two planes has been known since Sir 

 William Thomson's observations ; and it is well brought out 

 in the case of spheres, especially with the larger ones and at 

 small distances, that is, just where the disturbances are 

 smallest. I shall point out in the second part of this paper 

 that the breaking-stress may possibly depend, not only on the 

 state of the field at the point where the spark passes, but in 

 surrounding portions also. The further the spheres are apart, 

 the more nearly will all points of their surface reach the cri- 

 tical density at the same time ; and this, in the way to be 

 explained, may help the spark to pass. 



I feel much more doubtful as to the reality of the third 

 conclusion, that the explosive stress reaches a minimum with 

 increasing distances and then increases again; but on the 

 whole I believe the evidence to be in its favour. 



There are two causes which tend to increase the observed 

 difference of potential when the distance between the spheres 

 is larger. There may in the first place be an appreciable 

 leakage between the electrometer and the electrodes, and the 

 potential registered will in consequence be too high. That 

 leakage actually took place in some of Bailie's observations 

 is rendered probable by his observations on cylinders. For a 

 given surface-density on the inner cylinder the state of the 

 medium must be the same whatever the distance of the outer 

 cylinder; and there can be no reason therefore that the break- 

 ing-stress will depend on the diameter of that outer cylinder. 

 Gaugain arrived experimentally at the result which was to be 

 expected ; but Bailie's results, as given in Table V., show a 

 decided tendency towards a larger apparent normal force for 

 greater striking distances, the inner cylinder remaining the 

 same. This seems to me to point to a leakage effect. 



The second disturbing cause tending in the same direction 

 is the electrification of the leads. Bailie gives us no informa- 

 tion as to the disposition of his experiments and the size of 

 the leads, and seems to consider the difference of potential 

 between the electrodes to be the determining cause of the 

 spark and not the rate of fall at their surface. There is no 

 evidence in BaihVs paper that he realized the importance of 

 avoiding the presence of electrified surfaces in the neighbour- 



