170 



Prof. J. H. Poynting. Elective Current and [Feb. 12, 



electric induction towards the direction of motion of the unit length, 

 relatively to the tubes of induction. 



These principles are applied to special cases. 



A Straight Wire carrying a Steady Current. 



When a current C is in a wire, C induction tubes are supposed to 

 close in upon the wire per second, these being, as it were, broken up 

 or dissolved, their energy appearing finally as heat. 



This accounts for the constancy of current at all parts of the 

 wire in the steady state, in so far as it reduces thi-s constancy to a 

 particular case of the law, according to which there is the same total 

 induction over all sections of a tube. 



Also, since C tubes are broken up in the wire per second, and the 

 field is steady, C tubes must move inwards throug'h any curve encir- 

 cling the wire, and this will, by the third principle as modified, give 

 a line integral of magnetic intensity round the curve equal to AnrC. 



Since the electric intensity is not produced by static charges, we 

 must suppose it due to th« motion of magnetic tubes. If E is the 

 electric intensity, E magnetic tubes must cut any unit length parallel 

 to the axis of the wire per second moving inwards. 



On the supposition that the tubes bring their energy in with them, 

 it is shown that the electric and magnetic tubes each account for half 

 the energy, so that we may suppose that the energy crossing any 

 surface is equally divided between the two kinds. 



On the supposition that the tubes can be identified throughout 

 their motion in the insulating medium, their velocities are calculated. 



Discharge of a Condenser through a Fine Wire. 



When the terminals of a charged condenser consisting of two 

 parallel plates are connected by a wire, the energy which was before 

 the discharge chiefly between the two plates, now appears as heat in 

 the wire. The electric induction tubes are supposed to meve out- 

 wards from the space between the plates, keeping their ends upon the 

 plates or wires. They finally converge upon the wire and are there 

 broken up. The hypothesis is in accordance with the doctrine of 

 closed currents. For the total result is equivalent to the addition of 

 so many closed induction tubes to the circuit, the induction running 

 the same way relatively to the circuit throughout. When the con- 

 denser is discharged by imperfect insulation of the dielectric, we may 

 represent the process still as a closed current, the two parts of which 

 are the loss of induction and its dissipation, but this is -artificial, and 

 it is more natural to look upon the process as a decay of induction 

 without movement of fresh induction tubes inwards, and therefore 

 without the formation of magnetic induction. This case is discussed 

 at some length, as we can here realize what goes on at the source of 



