

Intelligence and Miscellaneous Articles, 451 



The tube laid round the wire neutralizes also its inductive action 

 in the entire outer space ; it forms a complete screen for the indu- 

 cing as well as the magnetic forces of the wire surrounded by it. 

 The screening action of the tube, according to this view, consists in 

 the fact that the actions of the central current are neutralized by 

 those of the induced current in the tube. 



This case is completely analogous to the electrostatic problem of 

 the distribution of electricity on two concentric cylinders, of which 

 the inner one is insulated and the outer put to earth. In like 

 manner the problem of the distribution of the current, even when 

 the wire and the tube are not concentric, and have other than 

 circular section, is to be solved by the analogous problem in elec- 

 trostatics. The screening action of the tube is complete even under 

 these altered conditions. 



The influence of the screening-tube on the velocity of propaga- 

 tion of waves in the wire results in the following manner : — By 

 restricting the magnetic field to the space between the wire and the 

 tube, the self-induction in the wire is considerably diminished. For 

 a circular section and concentric positions this diminution may be 

 easily given. An increase of the velocity of propagation corre- 

 sponds to this, if the capacity is almost the same, which, according 

 to electrostatic rules, is the case so long as the tube is insulated. 

 But if this is put to earth, the capacity of the inner wire is increased 

 in proportion as the self-induction is diminished, and consequently 

 the velocity of propagation remains unchanged. 



The capacity may, however, be considerably increased by filling 

 the space between the wire and the tube with a stronger dielectric 

 than air ; by interposing, for instance, a glass tube between the wire 

 and the tube. This glass tube has no influence on the self-induction, 

 but the capacity is almost doubled. The velocity of propagation 

 6inks thus in the ratio of V2 to 1. Sir W. Thomson has already 

 remarked this influence of the insulating medium on the velocity 

 of propagation. 



If a current is divided at two equal wires, one of which is sur- 

 rounded by a tube but the other not, the former will take a far 

 greater part of the current than the second, owing to the greater 

 diminution of the self-induction, which is due to the enveloping 

 tube. 



If, now, a second straight wire is placed parallel to a conductor 

 through which a current is passing, the primary current distributes 

 itself on the surface of the first, and the induced one on that of the 

 second conductor, in the same way as a given charge would be dis- 

 tributed on the first, and the induced one on the second conductor, 

 if the latter were put to earth. If the first conductor has a circular 

 section, the current is not distributed uniformly over its surface, but 

 has a greater density on the side nearest the second conductor than on 

 that away from it. If this condensation is visible, as in a Geissler's 

 tube, it presents the phenomenon of an attraction of the current 

 towards the approached conductor. As a matter of fact, there is 

 an electrodynamical repulsion between the conductor of the pri- 

 mary current and the approached conductor, which is observed at 



