PRESIDENT S ADDRESS—SECTION A. AT 
the wave length in a wire, as has been done by J. J. Thomson, 
and compare this with the wave length in air. By this method 
Thomson concludes that the velocity of propagation is the same 
in both cases in direct contradiction to the results obtained by 
Hertz. This is a point of considerable importance, as Maxwell’s 
theory clearly indicates that the velocity should be the same in both 
cases. Before we pursue the matter further, it will be convenient 
hereto give some slight comparative account of the different theories 
which are at our disposal if we abandon the theory of Maxwell. 
We shall then be in a better position to estimate the value of the 
evidence which is before us. Assuming that Hertz’s experiments 
have placed the existence of a medium beyond doubt, we need not 
devote any attention to those theories which depend on the 
assumption of action at a distance, and take no notice of inter- 
mediate effects. A good account will be found of them in a report 
of the British Association, 1885, by Professor Thomson. There 
are at least two theories besides Maxwell’s which claim our 
attention ; both of them take the action of a medium into account. 
One of them is due to Helmholtz, and the other—which is really 
the most general theory that can be framed from the experimental 
data—is due to J. J. Thomson. To come to the point at once, 
Helmholtz’s theory differs from Maxwell’s in making a rather 
more general assumption as to the relation between electric force 
and dielectric polarisation than is made by Maxwell. This leads 
to the polarisation currents being regarded as “incompressible,” 
while in Maxwell’s theory it is the “total” current made up of 
the conduction, and polarisation current which is mathematically 
so. Among other results to which the theory leads is that in 
some of the resonators used by Hertz, slight changes of capacity 
—as by adding or cutting off tinfoil—should not make much 
difference to the period, while the facts are that Hertz found that 
considerable difference was thereby produced ; on no theory but 
Maxwell’s is this accounted for. It may be mentioned that, as a 
working theory, Helmholtz’s is far more complicated than Max- 
well’s, so that unless it proved to possess any great superiority it 
could not be so serviceable. The general theory due to J. J. 
Thomson proceeds from the assumption that the dielectric polari- 
sation currents are proportional to the rate of charge of electro- 
motive force—we may say are equal to y times the electro- 
motive force. Now, if 7=K/4 7 we have Maxwell’s theory, and 
if 7=k K we have Helmholtz’s. The differences between this 
theory and Maxwell’s are summed up by Thomson as follows :— 
1, The existence of a normal wave in the general theory, but 
not in Maxwell’s. 
2. A difference in the velocity of propagation of the trans- 
verse wave. 
3. A difference in the relation between electric currents and 
magnetic force. 
