494 Proceedings of the Royal Society 
overshot the mark. The cause of the action appears to me as fol- 
lows ; — When the line of the armature (fig. 8) is vertical — when, in 
fact, the strongest action is taking place in the small coil — the wires 
of the large coil cut the lines of magnetic force between N and S 
at right angles, the best time and the best place for a current to he 
induced in them. Although, then, the longitudinal polarity of the 
iron has disappeared, the coil takes up the action and makes a north 
and a south end, even when the main line of the armature is up- 
right, and should be free from polarity. This coil induction or 
polarity is feeble, contrasted with that resulting through the iron, 
and would have little effect if the coils were near each other in 
size. It is only in the present case, where there is such a dis- 
parity between the coils, that the interference grows to a sensible 
amount. In support of this view of the matter, it may be men- 
tioned that when the larger coil is connected with the electro- 
magnet, little relief is felt on an easy 
circuit being made for the smaller coil. 
The effect of the interference is to lessen 
the current induced in the smaller coil. 
A particle at a, for instance (fig. 8), which 
when left to the action of the poles of the 
electro-magnet would give its full quota 
of electric induction, is by the cross polarity magnetically forced 
round, so to speak, into a less favourable position for doing 
so. But how is this interference stopped by a resisting external 
circuit? In this way, I imagine. The available electro-motive 
power may take the form of large quantity in an easy circuit, 
or little quantity in a resisting circuit. On consulting the 
galvanometer in a resisting circuit, while the strength is taxed 
to the utmost, the current is often found weak. It is the quantity 
of electricity that is the cause of the interference, and not the work 
value of the circuit. When the strength of the electric current is 
great with a resisting circuit, that of the magnetic current has been 
proportionally exalted. 
The interference of the two coils with each other can be shown 
in a simple way. When the coil of the electro-magnet is detached 
from the magnetic coil and joined up with a Bunsen cell, we have, 
on turning the handle, both armature coils prepared to give ex- 
