1891-92.] Prof. Knott and Mr Sliand on Magnetic Strains. 87 
The magnetising fields are given in magnetic units, and the 
changes of volume in the experimental scale unit. To reduce to 
cubic centimetres, multiply by 2‘23 x 10“® To reduce to dilatations, 
multiply by 6'5 X 10"^ 
Field. 
Volume 
Increment. 
Field. 
Volume 
Increment. 
Field. 
Volume 
Inerement. 
6-7 
-f 3-3 
81-5 
-52*8 
523 
4-59-8 
12-3 
+ 1-7 
100 
-24-9 
616 
4-57 
20-7 
- 0-3 
123 
-f 3-9 
701 
4-54-5 
27-4 
- 8-1 
127 
4- 8 
614 
4-56 
30-2 
- 9-5 
134 
-M2-5 
500 
-f 62 
37-5 
-22-3 
176 
4-35-3 
361 
4- 61 -5 
43-3 
-33-9 
181 
4-42 
377 
-f59 
53-6 
-50-2 
184 
4-37 
296 
4-55-5 
54-1 
-53-5 
191 
4-38-5 
180 
4-30-1 
51*2 
- 57 
301 
4-60-3 
143 
4-11*3 
64 
-66 
398 
4-61-8 
85 
-49 
67-2 
-63 
\ 
Here, as will be seen at a glance, the dilatation changes sign 
twice, first about field 20, and then about field 120. It attains a 
positive maximum about field 10, and again about field 400. The 
negative maximum is particularly sharp, and occurs about field 64. 
Comparing these volume dilatations with Mr Bidwell’s results for 
linear dilatations in the direction of magnetisation, we may pro- 
visionally draw these conclusions. The linear dilatation (/x) at 
right angles to the direction of magnetisation is generally of opposite 
sign to the linear dilatation (A) along the direction of magnetisation. 
At low fields the lengthwise elongation is numerically greater than 
twice the transverse contraction, so that the cubical dilatation 
(A + 2/x) is positive. Soon, however, as the field is taken stronger, 
the transverse contraction becomes numerically the greater, and the 
cubical dilatation changes sign. So long as the cubical dilatation 
remains negative, the transverse contraction has the advantage. It, 
however, seems to pass through a maximum and then diminish, so 
that the lengthwise elongation again recovers its superiority and the 
cubical dilatation becomes positive. Finally, since the cubical dila- 
tation remains positive up to fields much higher than that at which 
the change in length becomes contraction instead of elongation, it 
follows that the transverse contraction must also change sign and 
become a transverse dilatation. In a general way we may say that 
