32 PRESIDENTS ADDRESS—SECTION A. 
continuity in the cireuit—to close the circuit, as mathematicians 
say—which is the very essence of Maxwell’s theory. Poynting 
has recently shown that this theory requires us to imagine that 
the energy stowed between the condenser-plates moves out into the 
wire sideways, not through the armature-plates, as one would be 
apt to fancy. The formal proof of this is obtained on mathe- 
matical grounds based on certain consequences and further prin- 
ciples adopted in the theory; but we can see in a kind of way 
that it must be so. Imagine the conducting wires to be thick and 
long, but interrupted at their middle point by a short wire of 
high resistance. If the condenser is big enough the short wire 
will get appreciably heated by the passage of the current through 
it. Energy therefore has left the dielectric near the plates, and 
has converged on the short wire—at least for the most part. That 
very instructive experiment of the skeleton Leyden jar shows that 
the energy there, at all events, is in the dielectric. Moreover, we 
know that if we have a current in a wire energy is dissipated—while, 
from experiments on the induction of electric currents, we know 
that energy of current is stored to some extent in the dielectric. 
And further, currents flow in wires either as if they had no inertia 
—or are caused by “side” pull, not end thrust—and nobody has 
yet detected anything like inertia in the phenomena of currents. 
Moreover, we can show that energy stored in a dielectric which is 
undergoing rapid variation is propogated outward without any 
conductors at all, and consequently we are at least entitled to 
admit that there is no inherent improbability in Poynting’s 
deduction. It is verified of course, along with other results of 
the theory, in many ways, and more particularly has formed the 
subject of an experimental investigation by Hertz, the results of 
which are confirmatory in a very definite and striking manner. 
To render our ideas more precise it will be well to consider 
here the meaning of the phrase “quantity of electricity” in the 
light of the theory we are considering. To do this it is most 
convenient to commence with the conception of lines and tubes 
of force—an idea we owe in the first place to Faraday. The idea 
is simplicity itself. A line of force is any line drawn in the 
electric field in such a direction that a particle carrying a charge 
of electricity will move along the line if free to do so. Since the 
electric force at different points in the field will in general have 
different values, the further stipulation is made that in mapping 
a field we must draw lines in such a way that the number 
crossing unit area at any point will be proportional to the 
electrical force at that point. The lines are to be drawn close 
together where the field is strong, and far apart where the field 
is weak, and any small elementary space bounded as to its sides 
by lines of force we shall call a tube of force. Now it follows 
from the experiments of Faraday (and indeed these experiments 
gave rise to the theory we are discussing) that every tube of 
