4 
Proceedings of the Royal Society of Edinburgh. [Sess. 
pensating coils, measured as before the change of magnetic intensity at 
approximately the central portion of the wire. 
Experimental Methods. 
Cyclic Field. — After demagnetisation by decreasing reversals (the 
revolving commutator being used), the Held, put on by steps of increasing 
reversals, is reversed about forty times at some pre-arranged extreme 
value. One-half the average of two consecutive readings determines the 
induction at this cyclic extreme ; from which steps (the first being zero) 
are then taken to a sufficient number of points in the first and third 
quadrants from positive and negative amplitudes respectively, the induction 
change being noted. At each point the galvanometer readings are again 
noted, on the superposition of a series of nine damped trains of vibrations 
of increasing intensity, produced by increasing the heights through which 
the steel ball falls in arithmetical progression. On the completion of each 
series, the field is put off in steps of decreasing reversals. In no case are 
further vibrations superposed at the same or any other point until the 
demagnetising process, followed by reversals at the cyclic amplitude, 
has intervened. 
Cyclic Residual Magnetisation.— After about forty reversals at a high 
value of field amplitude, the induction is first determined as above 
described. Noting the galvanometer reading on withdrawing the field 
determines the residual magnetisation, which is now subjected to various 
values of a reversed field, so selected that when withdrawn the steel wire 
is left residually magnetised in zero field at a sufficient number of points 
of what may very well be called cyclic residual magnetisation between 
positive and negative maxima of residual magnetisation in the usual sense. 
A number of these points are shown in the reference diagram accompanying 
figs. III. and IV., and their positions on the vertical axis are determined by 
noting the two galvanometer readings when the reversed field is put “ on ” 
and "off,” taken in conjunction with the value of the residual magnetisation 
previously determined. At each point the galvanometer readings are 
again noted on the superposition of the nine damped trains of mechanical 
vibrations of increasing intensity produced as above described. On the 
completion of each series the cyclic extreme is again reverted to, and in 
no case are further vibrations superposed at the same or any other point 
until after forty reversals of the cyclic field, followed by the cyclic field 
process as above described. Owing to the high field amplitude, de- 
magnetisation by decreasing reversals was considered unnecessary. 
