294 
PROFESSOR J. H. POYNTING ON ELECTRIC CURRENT AND THE 
simple case of a slider AB, fig. 5, running on two parallel rails, AC BD, with a fixed 
cross piece CD, the tubes of magnetic induction running from above downwards 
through the paper. Let AB move so as to enlarge the circuit. We know from 
experiment that this tends to cause a current in the direction AC DB. 
Fig. 5- 
As AB moves through the field its motion tends to cause electric intensity in the 
direction BA. At the same time its kinetic energy is being continually converted 
into electric and magnetic energy which travels to the rest of the circuit there to be 
dissipated, that is, there must be a divergence of energy from AB. Instead then of a 
convergence of positive tubes running from B to A, we shall have what is magnetically 
equivalent—a divergence of negative tubes or tubes running from A to B, their 
motion outwards being accompanied by tubes of magnetic induction running round in 
the same way as if there were an ordinary current from B to A. These magnetic 
tubes must be supposed to move outwards in order to account for the direction of the 
electric intensity.* 
When these electric and magnetic tubes converge upon the rest of the circuit they 
will evidently form a current running in the direction AC DB. We have here taken, 
just as in the case of the condenser and the voltaic cell, the lessening of negative 
induction by its motion outwards, as equivalent to the increase of positive induction 
by its motion inwards, and we have considered both of them to indicate the appli¬ 
cation of electric intensity in the same direction in the conductor. 
If instead of considering AB as a whole we break it up into elements, each element 
will be a source of diverging negative tubes, and the remainder of AB will to that 
element be a part of the rest of the circuit. Hence some of the energy sent out 
from the element will converge on and be dissipated in AB, or AB will be heated just 
as the rest of the circuit. 
The general equations of the electromagnetic field. 
Wc can easily obtain equations corresponding to and closely resembling those ot 
Maxwell by means of the principles upon which this paper is founded. 
The assumption that if we take any closed curve the number of tubes of magnetic 
* [Added July 15.—The above must not be regarded as an attempt to explain the production of electric 
induction by the motion of a conductor in a magnetic field, but merely as an attempt to show bow the 
induction arising in the moving part of a circuit finds its way into the rest of the circuit.] 
