36 PRESIDENT’S ADDRESS—-SECTION A. 
‘‘ Whenever magnetomotive force is produced by change in the 
electric field, or by motion of matter through the field, the 
magnetomotive force per unit length is equal to 47 x the 
number of tubes of electric induction cutting or cut by unit 
length per second, the magnetomotive force tending to produce. 
induction in the direction in which a right-handed screw would 
move if turned round from the direction of the electric induction 
towards the direction of motion of the unit length relatively to 
the tubes of induction.” This in most cases may be simplified to 
the less general statement that ‘‘The magnetomotive force round 
any curve is equal to 4 + x number of tubes of electric induction 
passing in or out through the curve per second.” <A certain 
amount of experimental | verification has been given to this 
principle by the well-known experiment of Rowland. The 
experiment consisted in rotating a charged disc, and observing 
that this produced the same effect as would have been produced 
by an equivalent current. 
From these principles Maxwell developes certain consequences. 
which we will now discuss briefly. The first and most important 
consequence is that an electromagnetic disturbance will be propa- 
gated with a certain velocity through dielectrics, but will only be. 
propagated in a secondary manner, so to speak, through conductors. 
This comes about because (1) as has been shown the energy travels 
through the dielectric, and (2) because, according to the views of 
Faraday, a conductor is a body in which permanent electric 
induction or polarisation is impossible. When an electromotive 
force acts on a conductor we conceive it as first causing electric 
induction ; the nature of a conductor, however, must be such 
that it cannot support such a state permanently, and consequently 
there is something equivalent to a yielding, the field becomes 
discharged, and we have the phenomena of a current. Now the 
magnetic effect of a current is, I think, generally imagined as 
being in some way due to the process of yielding by the con- 
ductor, at least it is so in a model suggested by Maxwell, and 
elaborated by Oliver Lodge. It seems, however, that this view 
is at least unnecessary. The essence of the theory is that in 
dielectrics change of polarisation produces magnetic effects and 
consequently by analogy it is to the alternate setting up and 
breaking down of the field, or rather to the change of polarisation 
accompanying it, that we ought to look for an explanation of the 
magnetic action of the current. This involves, however, the 
assumption that it is only during either the setting up or the 
breaking down of the field that magnetic action is produced, so 
that the breaking down can not be a mere reversal of the setting 
up. It may be noted that to account for all the current magnetic 
effects in this way involves making the specific inductive capacity 
of conductors very high. A very instructive view of the pheno- 
mena of conduction has been explained by J. J. Thomson. We 
