CONDUCTION THROUGH GASES i6i 



electric force end on the surfaces of conductors, 

 and the opposite ends of each tube, where they 

 touch the conductors, constitute unit electric 

 charges of opposite sign. The state of strain in 

 the field Is such that we must imagine the tubes 

 of force as tending to shorten in length and to 

 push each other apart ; and, when the poles of 

 a battery are disconnected, the tubes of force 

 will be in equilibrium under these forces. The 

 distribution of the electric tubes will then be 

 very similar to that of the magnetic lines, made 

 visible by the filings shown in Fig. 31 on p. 153. 

 A conducting wire must be regarded as a 

 channel along which the free ends of a line or 

 tube of force can move, and, when the poles of 

 the battery are connected by means of a wire, the 

 tubes of force in the surrounding air run their 

 opposite ends on to the wire, pull those ends 

 towards each other, and shut up. Other tubes 

 are then pushed into the wire by their mutual 

 transverse pressure, and are obliterated in turn. 

 The tubes of force In the dielectric field are thus 

 inclined to disappear, and the state of sethereal 

 strain In that field tends to be relieved. Simulta- 

 neously, however, the battery endeavours to 

 reassert the original distribution of tubes, and 

 once more to set up the strain. In this way 

 new tubes are constantly forming between the 

 terminals of the battery, and are as constantly 

 pushed into the connecting wire, where they 

 vanish. When the connection is metallic, it is 

 only the negative ends of the tubes, attached to 

 the electrons, that move, the positive ends 

 remain at rest. If, on the other hand, part of 

 the circuit is composed of an electrolyte, In that 



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