146 
Journal of the Mitchell Society [December 
in the other trough grows larger and larger until the velocity is that of 
light, when the streams of balls in the two troughs are exerting 
zero fource on each other, (for their electrostatic repulsion is 
exactly balanced by their electromagnetic attraction), and yet 
they are said to be behaving like electric currents. Why, then, 
do parallel currents actually attract each other? No one supposes 
that a current in a wire travels faster than light. Some years 
ago in Cambridge I asked Prof, (now Sir) .J. J. Thomson about it, 
and he replied that my analogy was all right, except that according 
to the electron theory the glass trough should have a metal covering 
outside, which is positively charged, the hopper should be negatively 
charged, and the charge on a unit’s length of the trough should 
equal the sum of the negative charges on the balls contained in 
that length. Then, the analogy, while crude, would be com- 
rlete:, — the steel balls would represent electrons, and the current 
in the ordinary sense would flow up the trough instead of down. 
The charge on the covering of the trough would represent the 
the charges on the positive atoms in a conductor. Under these 
circumstances it is easy to see that attraction between the troughs 
would ensue as soon as the balls began to move. Prof. Nipher’s 
explanation, therefore, would seem to be valid only on the suppo- 
sition that the positive ions in the line of the disruptive discharge 
(which are dashing towards the negative terminal) would take 
the place of the metal-covered trough in my analogy, thus render- 
ing the electromagnetic attraction of the moving electrons effective 
in drawing them together in a column which continually thins 
out towards the positive terminal. If this be true, the effect 
ought to be rendered more intense because of this consideration : — 
the analogy would then be that of the trough itself (carrying a 
a positive charge) moving in the opposite direction to the motion of 
the steel balls , thus making the relative velocity of the balls greater 
and the attraction more intense. 
But there is another way of looking at it which may be more 
natural. The negative terminal is a large sphere 10 cm. in diameter, 
while the positive terminal is but 1 cm. in diameter. The lines 
of force are therefore strongly convergent frem the negative to the 
positive sphere, somewhat like the ropes from the gas bag of a 
