PRESIDENT’S ADDRESS—SECTION A. 541 
between the plates is not only proportional to the electric 
intensity, but is numerically equal to the product of the electric 
intensity by the factor 7a where K is the specitic inductive 
capacity of the medium. In order to examine the nature of this 
supposition, we will for a moment take up another point of view 
and consider, as was formerly done, that there is a substance 
called electricity, and that plates are electrified when this 
substance is distributed over them. Maxwell’s assumption 
amounts—in the language of this theory—to making the charge 
on the plates exactly equal to the loss of electricity by the 
dielectric, or, in other words, makes electricity incompressible. 
This point is important, because it distinguishes Maxwell’s theory 
from the theory of Helmholtz, and from other theories in which 
the action of the medium is taken into account. Reverting to 
the theory proper, let us see what Maxwell’s views are as to 
discharge. The dielectric returns to its normal condition, and 
we have a current in the wire, and, moreover, says Maxwell, 
there is a falling back of the polarisation to its neutral state. If 
we differentiate the polarisation with respect to time, we have a 
quantity which may be expressed in words as the time rate of 
change of the polarisation, and this, according to Maxwell, forms 
a polarisation current, and produces just exactly similar magnetic 
effects to the magnetic effects produced by the conduction current 
in the wire. I say similar, for in any condenser discharge these 
effects are on a much smaller scale. Thus during a discharge the 
energy available to produce magnetic effects due to the rate of 
change of polarisation in the dielectric will be about a thousand 
million million million times less than the corresponding magnetic 
energy of the current in the wire. Such small effects have 
hitherto escaped direct detection, and the proof of their existence 
must therefore, for the time being, remain indirect. There is such 
a proof, however, and that of a most satisfactory character, as we 
shall see when we consider the meaning of some recent experi- 
ments, of first-rate importance, due to Hertz. I may mention 
that I, as well as others, have calculated the possibility of obtaining 
evidence of these polarisation currents—or rather, and this is the 
essential point of the theory—of their magnetic action, and find 
that, thanks to the properties of quartz threads, there is just a 
possibility of their detection. The difficulty lies not so much in 
obtaining evidence of the existence of the minute couple we should 
have to observe, but in seperating the action we are in search of 
from others due to real conduction in the dielectric or, most of 
all, to small magnetic effects. 
The real current, then, in Maxwell’s theory is made up of the 
conduction current in the wire and the small current due to change 
of polarisation in the dielectric. It will be observed that this 
small current is to be taken into account in order to avoid dis- 
