250 Proceedings of Royal Society of Ediiiburgli. [sess. 
iron. In every case, Tube V. excepted, the phenomenon did not 
declare itself when the tube was treated for the first time. Under 
this condition, the dilatation began negative in the lowest fields. 
It was only after the tube had been left residually magnetised by 
the action of a larger field that the application of a very small field 
produced this positive dilatation. The result is, in fact, a particular 
case of a more general phenomenon. It is a case of magnetic after- 
effect. 
The phenomenon, as it is shown with Tubes I., II., III., and 
IV., may be briefly described as follows : — 
When a smaller field is applied after a larger field has been 
applied and removed, the contraction of volume is less when the 
successively applied fields are in the same direction than when they 
are in opposite directions. When the second field is small enough 
the contraction changes sign, becoming a positive dilatation when, 
and only when, this field has the same direction as the previously 
applied stronger field. For, when the small field is applied in the 
reverse direction to the previous strong field, this change of sign 
never takes place. It is obvious, indeed, that in magnetically 
unbiassed iron the change of volume should be exactly the same in 
whatever direction the field is applied. Flow whenever, in the 
case of the tubes specified, this positive dilatation for low fields 
showed itself, the mere reversal of field produced a negative dilata- 
tion. Such a change of sign with the field demonstrated a magnetic 
bias. And even when the field was not low enough to produce this 
positive dilatation, an exactly similar magnetic bias was indicated 
by the different contraction produced when the field was simply 
reversed. To get rid of this bias, due to the after-effect of a 
previously applied stronger field, it was sufficient to subject the iron 
to a succession of gradually diminishing fields, alternating in direc- 
tion. This method of neutralismg^ by reversals left the iron in an 
approximately unbiassed condition, and quite destroyed the tendency 
to positive dilatation in low fields. 
With Tube V., however, positive dilatation always showed itself 
in certain low fields. That this was not due to magnetic bias, was 
proved by the fact that the reversal of the field failed to change the 
sign of the dilatation. 
* “ Demagnetising” by reversals, it is usually but unfortunately called. 
