282 PROFESSOR J. H. POYNTTNG ON ELECTRIC CURRENT AND THE 
continually-increasing induction, and breaks the tubes up, as it were, their energy 
appearing finally as heat. 4 ' 
Fig. 1. 
Let us see how this hypothesis accounts for known facts, when aided by the two 
principles just laid down. 
It accounts at once for the constancy of the current at all parts of the wire in the 
steady state, in so far as it reduces this constancy to a particular case of the law 
according to which there is the same total induction over all cross sections of a tube. 
If, for instance, there were more induction entering at A than at B, then more tubes 
must be entering at A, and so there would be an increase in the number of tubes 
left in the medium about B, or the field would not be steady. 
Further, if we draw any closed curve embracing the wire once, we may apply the 
third principle to give us the line integral of the magnetic intensity round the curve. 
For this is a case where change is certainly going on in the electric field, and the 
magnetomotive force is due to this change. The field being steady, if C tubes enter 
the wire and are there broken up, C tubes must cross through any encircling curve to 
supply their place, or the line integral of the magnetic intensity round the curve is 
equal to AnX number of tubes passing through the boundary per second, i.e., 47tC. 
If the curve be a circle of radius r, with its centre in the axis and plane perpendicular 
thereto, the intensity at any point of this circle will be tangential to it, and equal to 
4tt0 20 
2 7 rr r 
The known constancy of the line integral of the magnetic intensity round the wire, 
which the hypothesis thus accounts for, almost seems to force the hypothesis upon us, 
* May we not say that the tubes are dissolved. The term seems to suggest that the induction is not 
destroyed, but only loses its continuity. Probably this is the case; for on the electromagnetic theory of 
radiant energy, when the wire is heated, it sends out the energy it received, again in the electro¬ 
magnetic form. 
i, 
