of Electricity through Gases. 199 



Mr. Jaiimann comes to the conclusion, which appears well 

 supported by the evidence he gives, that the potential required 

 to produce a spark is the greater the more slowly the potential 

 is raised to its final value. It' the potential is raised quickly, 

 so that, according to our view, no time is given for the par- 

 ticles to move along the surface of the conductor, the spark- 

 potential has much smaller values than those usually given. 



I am not very confident that a satisfactory explanation of 

 the observed effects of curvature can at present be given. If 

 a surface-layer such as I imagine really exists, forces of the 

 nature of surface-tensions may modify its properties. But it 

 is also possible that the density of the layer may depend, not 

 only on the inequality of the field at different points of the 

 surface, but also on the inequality of the field along the nor- 

 mal to the surface. Particles may be drawn by the electric 

 forces from the gas into the surface-layer whenever there is a 

 rapid variation of the field along the normal. If the polarized 

 particles point in the direction of the lines of force, they will 

 tend to move along them under the action of a force propor- 



tional to -j-3, where Y is the potential ; and this quantity is 



the larger for the same value of R the greater the curvature 

 of the surface. It is possible that this is the explanation of 

 the greater spark-potential required for small than for large 

 spheres. The two effects combined may explain the fact 

 that when the spheres are moved away from each other, the 

 breaking-stress at first diminishes and then increases again. 

 For small distances the inequality of the field along the sur- 

 face causes a crowding of the molecules, and for large distances 

 the inequality of the field along the normal produces the same 

 effect. At intermediate distances it is probable that either a 

 maximum or a minimum of .the normal force should take 

 place, but which it should be it is impossible to predict theo- 

 retically at present. 



I do not attach very much importance to some of the sug- 

 gestions I have made in order to explain a quantity of very 

 puzzling facts ; but it seemed to me to be worth while to place 

 them before the public, as others may be induced to join in an 

 experimental investigation which promises to yield results of 

 importance. 



I have to thank Mr. H. Holden for much assistance in the 

 numerical calculations contained in this paper. 



