332 
MR. C. CHREE OX THE EFFECTS OF PRESSURE 
On breaking the second currents the rods all returned to the length they possessed 
on the break of the first currents, so that the chancres in the leng'ths of the rods 
became cyclic after a single make and break of the current. The last three experi¬ 
ments are, it will be observed, very far from agreeing with Joule’s laws. 
The experiments suggest that it is mainly on the intensity of the existing 
magnetisation that the phenomena depend, but the divergence between the figures in 
the second and fourth columns indicates that the previous history of the material 
may, in some cases at least, be of considerable importance. 
In all the iron rods the induced magnetisation is clearly much below the critical. 
In 7, the softest of the steel rods, the induced magnetisation though below is clearly 
approaching the critical. In rod 8, the critical magnetisation lies between the 
induced and the residual; and if a critical magnetisation existed in the case of rod 9 
it must have been less than the residual. 
§ 5. Of Mr. Shelford Bidwell’s experiments those described in the ‘ Philosophical 
Transactions,’ appear the most complete. In these, he first demagnetised the specimen, 
and then sent through a surrounding spiral a series of currents gradually rising in 
strength. For each strength of current the alterations in the length of the rod 
accompanying two successive makes were observed and recorded. The Tables III. 
and IV., pp. 220 and 221, for an iron rod show a small but unmistakeable difference 
between the effects of the first and second makes of a current of given strength. 
In Table V. p. 222, for a cobalt rod the differences might well be attributed to 
experimental errors. 
Mr. Shelford Bidwell found critical fields both for iron and cobalt. Iron he 
found to lengthen or shorten when magnetised according as the magnetising field was 
below or above the critical, while cobalt presented exactly the opposite phenomena. 
A nickel rod shortened in all fields up to 1400 C.G.S. units, and the experiments 
left the existence of a critical field an open question. The critical fields for several 
specimens of iron varied from about 270 to 380 C.G.S. units, and the maximum 
elongations appeared in fields of about 80 C.G.S. units. In one cobalt rod the 
maximum shortening occurred in a field of about 400 C.G.S. units, and the critical 
field considerably exceeded 800 C.G.S. units. A second softer specimen showed a 
maximum shortening in a field of about 300 C.G.S. units, and had a critical field 
of 750 C.G.S. units. In these experiments, the rods were free from stress. Mr. 
Shelford Bidwell’s experiments, however, described in the ‘ Proceedings of the 
Ptoyal Society,’ show that tlie critical fields for iron are lowered by tension, a result 
in exact accordance with Joule’s experiments. 
§ G. Professor Barrett found a cobalt rod to lengthen when exposed to a com¬ 
paratively weak field. The data he supplies are too limited to enable one to judge 
whether his later experiments on the subject were wholly free from the objections 
which he himself believed to have invalidated the conclusions based on his earlier 
experiments. I scarcely think his results afford any reasonable ground for doubting 
