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PROFESSOR J. H. POYNTING ON ELECTRIC CURRENT AND THE 
Modification of the Second Principle. 
I propose to replace the second principle by the following -- -Whenever electromotive 
force is produced by change in the magnetic field, or by motion of matter through the 
field, the E.M.F. per unit length or the electric intensity is equal to the number of tubes 
of magnetic induction cutting or cut by the unit length pier second, the E.M.F. tending 
to produce induction in the direction in which a right-handed screw would move if 
turned round from the direction of motion relatively to the tubes towards the direction 
of the magnetic induction.* 
In order that the results obtained from this should agree with those obtained from 
Maxwell’s statement of the principle, it is necessary that change in the total 
quantity of magnetic induction passing through a closed curve should always be 
produced by the passage of induction tubes through the curve inwards or outwards. 
In some instances this is undoubtedly the case, as, for instance, where a part of a 
circuit moves so as to cut a fixed magnetic field, or where a magnet moves in the 
neighbourhood of a circuit. Here the E.M.F. is equal to the number of tubes cut by 
the wire per second, and its seat is that part of the wire cutting the tubes. In other 
cases, as, for instance, where the wire is between the poles of an electromagnet whose 
magnetising current is changing, we have no direct experimental evidence of the 
movement of the induction in or out. But the induction tubes are closed, and to 
make them thread a circuit we might expect that they would have to cut through the 
boundary. The alternative seems to be that they should grow or diminish from 
within, the change in intensity being propagated along the tubes. This would be 
inconsistent with their closed nature, unless the energy were instantaneously pro¬ 
pagated along the whole length, and is further negatived by the theory of the 
transfer of energy, which implies that the energy flows transversely to the direction 
of the tubes. I shall suppose, then, that alteration in the quantity of magnetic 
induction through a closed curve is always produced by motion of induction tubes 
inwards or outwards through the bounding curve. 
* Taking the electric intensity as always perpendicular to the plane of motion of the magnetic tubes 
through a point, and equal to the number cut per second by unit length of the normal to the plane of 
motion, we can easily show that the component of the intensity in any other direction will be equal to 
the number of tubes cut by a line of unit length in that direction. For let OA represent a small length 
drawn perpendicular to the plane of motion, and let OP represent a line drawn in any direction making 
0 with OA. Draw AP perpendicular to OA, and meeting OP in P. Then the same number of tubes will 
cut both OA and OP, since AP is parallel to their plane of motion. If the number cutting OA be 
E X OA, where E is the number cutting unit length, and therefore equal to the resulting intensity, the 
number cutting unit length of OP will be ® = E cos or th e component of the intensity along OP. 
