126 REPORT—1885, 
an electrified body was supposed to contain a certain quantity of some- 
thing called electricity, rules were given for measuring this quantity, 
and the phrase ‘quantity of electricity’ meant something quite definite, 
Tn Maxwell’s theory, where everything is referred to the dielectric, the 
meaning of the phrase is not so obvious. We can, however, arrive at 
some idea of what is meant by the consideration of what are called ‘tubes 
of force.’ Let us suppose at first that the dielectric is air. A line of 
force is a line whose direction at any point coincides with the direction of 
the electromotive force at that point, so that we may conceive the electric 
field to be filled with lines of force. If we consider the lines of force 
passing through some small closed curve, they will form a tube, and such 
a tube is called a tube of force ; and if the dimensions of the tube are such 
that the product of the cross section at any point and the electromotive 
force at that point is constant and equal to 47, the tube is called a unit 
tube. We may thus conceive space to be filled with unit tubes of force. 
Since the electromotive force inside a conductor vanishes these tubes will 
end at the surface of a conductor. And the quantity of electricity on the 
conductor will be equal to the excess of the number of lines of force which 
leave the conductor over those which enter it. A tube is said to leave the 
conductor when the direction of the electromotive force is along the normal 
drawn outwards, and to enter it when the direction of the electromotive force 
is along the normal drawn inwards. As the conductor moves about it may 
be supposed to carry the tubes of force along with it, so that the number of 
tubes which end on the conductor remains constant. This way of look- 
ing at electrification is quite satisfactory as long as we keep to one 
dielectric air; when we have to consider different dielectrics it requires 
modification, because the electromotive force changes abruptly as we pass 
from one dielectric into another, so that a tube which was a unit tube in 
one dielectric is not so in another. It is easy, however, to extend the 
definition of unit tubes so as to meet this difficulty ; for if the tubes pass 
from one dielectric A into another B the ratio of the product of the cross 
section and electromotive force is constant for all the tubes and depends 
only on the nature of the dielectrics ; this ratio is the ratio of the specific 
inductive capacities in Band A. Air is taken as the standard dielectric, 
and the specific inductive capacity of another dielectric A is the ratio of 
the product of the electromotive force and cross section of a tube in air 
to the product of the same quantities for the same tube in the dielectric 
A. Thus if we amend our definition and say that a circuit tube is one 
such that the product of the cross section, the electromotive force, and the 
specific inductive capacity of the medium in which the cross section is 
situated is equal to 47, then the quantity of electricity on a conductor is 
equal to the excess of the number of unit tubes which leave the conductor 
over the number of those which enter it. In this way we get an idea of 
what is meant by ‘ quantity of electricity’ in Maxwell’s theory. Maxwell 
accounts for the forces observed between electrified bodies by a system 
of stresses in the dielectric separating them ; as, however, at present we 
wish to compare Maxwell’s theory with other theories which do not 
touch upon this point, we shall discuss this part of the theory separately 
later on and go on to discuss those points which are involved in all the 
theories. 
The next great point in Maxwell’s theory is the development of 
Faraday’s remark that the electrotonic state may exist even in non-con- 
ductors, 7.e., that the dielectric surrounding a changing current is acted 
