ON THE MAGNETISATION OE COBALT. 
331 
induced in the bar, and that on the break of the current there remains an increase in 
length proportional to the square of the intensity of the residual magnetisation. 
Joule also found that iron and soft steel under tension lengthened or shortened when 
magnetised according as the tension was small or great. He believed the shortening 
proportional to the product of the bar’s mcignetisation into the strength of the 
magnetising current. This last conclusion, however, is certainly not warranted by 
the results of his experiments. 
On Joule’s p. 237, are given the results of an experiment, numbered 23, in which 
a soft steel wire exposed to a certain tension showed an increased length while 
exposed to a certain current, lengthened further when the current was broken, and 
then shortened on the make of a stronger current. This suggests that for the wire in 
question under a given load there existed a critical intensity of magnetisation, and 
that the bar lengthened or shortened according as the magnetisation was less or 
greater than the critical value. The true interpretation of this experiment seems to 
have been entirely missed by Joule, and it would not appear to have attracted the 
notice of subsequent observers. It is thus only within the last few years that the 
existence of a critical field, or state of magnetisation, has been brought to light by the 
decisive experiments of Mr. Sheleord Bidwell. 
§ 4. Mayer experimented with rods free from longitudinal stress, and he gives the 
results of the make and break of only one strength of current. The current was, 
however, twice made and broken in some at least of the experiments, and the results 
of both the magnetic cycles are recorded. Some of the results, given in a table on 
his p. 200, are so suggestive in the light of our present knowledge that I reproduce 
them here in a form slightly altered to suit the present inquiry. Tlie numbers in the 
first column distinguish the rods used, all the other numbers give the lengthening, or 
when with a minus sign the shortening, of the rods in terms of an arbitrary unit. 
Increa.se in length of rods. 
Bod No. 
First make of 
circuit. 
After first break. 
Second make of 
circuit. 
a 
1-15* 
•4 
11 
1 2 
1-6 
•4 
1-6 
Iron. 
_»_ 
CO 
2-0 
11 
2-5 
2-5 
I'.SS 
2-5 
i 5 
1-65 
1-05 
2-05 
L6 
1-4 
•55 
1-45 
r? 
•8 
1-4 
115 
I<8 
~ -25 
•25 
- -25 
^ 1 9 
- -4 
- -15 
- -35 
* In original 1'25, but this seems a misprint. 
2 u 2 
