286 
PROFESSOR J. H. POYNTING ON ELECTRIC CURRENT AND THE 
If the current-bearing wire is copper. E,= —p, and with g=l the velocity becomes 
1642-r 
2tt« 2 
We cannot assign a velocity to the electric tubes within the wire since the number 
is diminishing as their energy dissipates. But the magnetic tubes crossing unit length 
parallel to the axis are still unchanged in number, so that we may assign a velocity to 
them. This velocity means that with the known value of the magnetic induction this 
velocity will give the number crossing inwards required to produce electric intensity E. 
E a? 
The velocity will be found equal to _-y— or 
In the case of a copper wire this becomes 
R a? 
2 fir 
1642 
2/jbTrr 
Discharge of a condenser through a fine wire. 
Let us suppose that we have a condenser consisting of two parallel plates A and B 
and charged with equal and opposite charges. Then we know that there will be 
electric induction between the two plates, and that according to Maxwell’s theory 
the energy of the system is stored there. We may form an idea of the distribution 
of the energy by drawing the unit induction tubes, each starting from and ending in 
unit quantity of electricity, and dividing these into unit cells by the level surfaces, 
drawn at unit difference of potential (fig. 2). If the dimensions of the plates be 
Fig. 2. 
great compared with their distance apart, then nearly all the cells will be between 
the two plates, and since each cell contains half a unit of energy, nearly all the energy 
is there. There will, however, be slight induction, and therefore some small quantity 
of energy in the surrounding space. 
