404 Proceedings of Royal Society of Edinburgh. [sess. 
Curve (a) corresponds to the permanent magnetic cycles (1), (3), 
(5); curves ( b ) and ( c ) give the cycles when the linear current is 
flowing first in the one and then in the other direction. The 
current used in this experiment was about 2 amperes. 
A glance at these curves is suffleient to teach us that the linear 
current modifies the properties of iron in relation to the magnetic 
after-effect in three well-marked ways. 
First , the total range of magnetic intensity produced by a given 
cyclic variation of magnetic force is markedly diminished when the 
current is flowing along the wire. This means a diminution in 
susceptibility. Here also may be mentioned the fact that the first 
effect of putting on the current when the wire is strongly magnetised 
is a diminution of longitudinal magnetic intensity. 
Secondly , when the linear current is flowing, the average magnetic 
intensity over a whole cycle no longer corresponds to the condition 
of zero polarity, as in the normal case, when no linear current is 
flowing. For the one current the magnetic intensity oscillates, so 
to speak, about a large positive polarity ; and, for the current in the 
reverse direction, it oscillates about a nearly equal negative polarity. 
If we reckon polarity in the usual way, as a directed quantity 
measured from the south pole to the north, then the direction of 
the linear current is in the same direction as the average polarity 
which it sustains, when the wire is subjected to an absolutely 
symmetrical cycle of magnetising force on each side of zero. 
Thirdly , the curves (b) and (c) are no longer symmetrical on the 
side of the zero line of magnetic force. The closed curve (b) is 
more pointed at its positive end than at its negative end; the 
curve (c), on the other hand, is more pointed at its negative end 
than at its positive. If we turn the figure upside down, the 
general appearance remains the same as before. Thus, the effect 
of the linear current is such as very distinctly to modify the form 
of the ascending and descending branches. The descending branch 
of (b) is very similar to the ascending branch of (c) ; and the 
'ascending branch of ( b ) is very similar to the descending branch 
of (c). We may connect this peculiarity with the peculiarities 
already described in these words : — 
A current passing along an iron wire, which is being magnetised, 
diminishes the apparent susceptibility of the wire ; but this effect 
