ON ELECTRICAL THEORIES. 123 
tial energy, because that at any time depends only on the position of the 
system at that time ; it is not kinetic, because that depends only on the 
position and velocity of the system at the time under consideration, 
whilst Neumann’s energy depends on the velocity and position of the 
system at some previous time. In spite of all this, however, Neumann 
applies the ordinary dynamical processes to this energy just as if it were 
kinetic or potential ; and in this way arrives at the same expression as: 
Weber for the force between two moving electrified bodies. The rest of 
the theory is the same as Weber’s, except that Neumann’s assumption 
about the nature of a current is different from Weber’s. According to 
Weber, an electric current consists of equal quantities of positive and 
negative electricity, moving with equal velocities in opposite directions. 
According to Neumann, the positive electricity alone can move, the nega- 
tive being attached to the molecules of the conductor. Riecke and Clausius 
have shown that with this assumption and Weber’s law a steady current 
must exert a force upon a particle at rest and charged with electricity, and 
must in consequence produce an irregular distribution of electricity over 
any conductor in its neighbourhood. 
Theories which are founded on dynamical considerations and which take 
into account the action of the dielectric. 
In the theories we have hitherto considered, the influence of the 
medium which exists between the currents has been left altogether out of 
account. In the theories which we shall now proceed to discuss, the in- 
fluence of this medium is taken into consideration. This is, perhaps, the 
most important step that has ever been made in the theory of electricity, 
though from a practical point of view it is comparatively of little import- 
ance; in fact, for practical purposes almost any one of the preceding 
theories will satisfy every requirement. 
Faraday was the first to look upon the dielectric as an important 
agent in electrical phenomena; he was led to this by his desire to get rid, 
as far as possible, of the idea of action at a distance, which was so pre- 
valent in his time, but to which his researches have given the death-blow- 
In his ‘ Experimental Researches,’ § 1164, speaking of electrostatic in- 
duction, he says, ‘I was led to suspect that common induction itself 
was in all cases an action of contiguous particles, and that electrical 
action at a distance (7.e. ordinary inductive action) never occurred except 
through the influence of surrounding matter.’ And later on he gives his: 
views as to the nature of the effect in the medium; in § 1298 of the 
‘Researches’ he says, ‘Induction appears to consist in a certain 
polarised state of the particles into which they are thrown by the electri- 
fied body sustaining the action, the particles assuming positive and 
negative points or parts, which are symmetrically arranged with respect 
to each other and the inducting surfaces or particles. This state must 
be a forced one, for it is originated and sustained only by force, and 
sinks to the normal or quiescent state when that force is removed. It 
can be continued only in insulators by the same portion of electricity, 
because they only can retain this state of the particles.’ He gives an ex- 
perimental illustration of his view in §1350. He says, ‘ As an illustration 
of the condition of the polarised particles in a dielectric under induction 
I may describe an experiment. Put in a glass vessel some clear rectified 
