450 Prof. J. J. Thomson on the Discharge of Electricity 



region A which it shields. This region has to be shielded 

 from tubes of electrostatic induction coming from the left, 

 which have to pass through the shield before reaching A, and 

 from tubes coming from the right which have to pass through 

 A before reaching the field. The action of the shield in the 

 first case is very simple, for when a tube gets inside a con- 

 ductor it at once attempts to contract to molecular dimen- 

 sions, and after a time proportional to the specific resistance 

 of the conductor it succeeds in doing so. Thus if the shield 

 is made of a good conductor the tubes of electrostatic induc- 

 tion will be transformed into molecular tubes before they 

 have time to get through ; so that the shield will protect A 

 from all tubes which have to go through it. The way the 

 shield destroys or rather neutralizes the effect of the tubes 

 coming from the right is somewhat different : when a positive 

 tube reaches the shield a negative one emerges from it, tra- 

 veiling at right angles to itself in the opposite direction to 

 the incident tube ; thus when the first few tubes reach the 

 shield from the right they will produce a supply of negative 

 ones, and the presence of these negative tubes at A concur- 

 rently with the positive ones which continue to arrive there 

 will weaken the field to a greater and greater extent as A 

 approaches the shield. At the surface of the shield itself the 

 neutralization will be complete. A dielectric whose specific 

 inductive capacity is greater than usual will behave in a 

 similar way to a metal plate, but to a smaller extent. It will 

 emit tubes of the opposite sign, but not so numerous as those 

 incident upon it. Thus a metal plate, or even one made of a 

 dielectric of considerable specific inductive capacity, will 

 reduce very considerably the tangential electromotive force 

 on either side of it. 



I have made several experiments in which this effect was 

 very strikingly shown. In one of these, two square discharge- 

 tubes of equal cross section placed near and parallel to each 

 other were connected by a cross tube, so that the pressure 

 was the same in both tubes ; a fine wire passed round the in- 

 side of one of the tubes, its ends being connected together so 

 that it formed a closed circuit, the other tube contained 

 nothing but air at a low pressure. When this double tube 

 was placed outside the primary the discharge went, at the 

 passage of each spark, through the tube without a wire, while 

 the tube containing the wire remained quite dark. A similar 

 experiment was tried by taking a cylindrical tube and sus- 

 pending in it a metal ring coaxial with the tube ; in this case 

 it was easy to adjust the spark-length so that no discharge 



