PRESIDENT’S ADDRESS—SECTION A. a3 
force must start at a positively charged surface and end at a 
negatively charged surface. A property of such tubes is that the 
product of electric force by area of section is constant throughout 
the tube. Now we may define unit quantity of electricity as the 
electrification which causes the sum of the above products for all 
the tubes drawn in connection with it to have the value 
4-7. Looking at it in a different way, we may say that a tube 
for which the above product is 4 7 is a unit tube and corresponds 
to a unit amount of electrification. From what has been said, 
however, it is clear that since a dielectric, such as sulphur say, 
has rather more pronounced electrical properties than air, we 
shall have rather to extend our definition if it is to fit the case of 
a condenser, between whose plates there is a layer of sulphur as 
well as a layer of air. The electric force must abruptly change 
at the boundary of the media, and consequently what was a unit 
tube in air will no longer remain a unit tube in sulphur. All 
the tubes, however, will sutter alike, and since the forces, other 
things being equal, depend on the specific inductive capacity of 
the dielectric, we will amend our definition to the extent of 
introducing the factor K, so that our unit tube must be one in 
which the product area of section by electric force by specific 
inductive capacity is equal to 4 z. Now the value of K for air 
is taken nearly as 1, and consequently the amount of energy of 
electrification required to set up a unit tube will really be a sort 
of absolute measure of that electrical property, which, reckoned 
with respect to a vacuum, is denoted by a value nearly unity. 
Looking at it in another way, as it is important we should see 
the matter clearly, let us suppose that two insulated particles 
are immersed in a medium at unit distance apart, and electrified 
till they exert unit force on one another. If we imagine the 
particles held in position by elastic springs, and then displace the 
air in which the particles have been immersed by benzene or any 
other dielectric, the springs will be observed to relax, shewing 
that the force is not so great as it was. If we wish to get the 
springs back to their original state of strain we shall have to 
work our electrical fnadhine again to increase the electrification. 
Hence it is clear that the amount of electrification indicated 
by one unit depends on the specific inductive capacity of the 
medium in which the experiment is supposed to be made. 
Returning to Poynting’s statement of the mode in which the 
energy of a charged condenser gets into a wire, we see that it is 
equivalent to the statement that the tubes of force move out 
from the dielectric near the plates and converge on the wire 
where they give off their energy. This, of course, involves what 
may be called an assumption, namely, that energy is transferred 
continuously, and is not destroyed at one point and re-created at 
another. That this is really the case will be shown later on. 
The next great principle of Maxwell’s theory refers to the 
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