552 
PROFESSOR J. A. EWING ON EXPERIMENTAL 
(10) Pianoforte steel at normal temper (fig. 11) : — Ergs. 
For the double reversal.J3>d«!p =116,000 
For the small loop.= 3,500 
(11) The same, annealed (fig. 12) :— 
For the double reversal.= 94,000 
For the small loop.— 5,000 
(12) The same, glass-hardened (fig. 13) :— 
For the double reversal. J=117,000 
For the small loop.J3c£<§ = 2,500 
The following measurements are taken from the experiment of § 31, fig. 14, in 
which an iron wire was subjected to a large cycle of magnetisation first in the 
annealed state, and then again after being stretched considerably beyond its limit of 
elasticity. The values of jpdjo (for a double reversal of the greatest intensity of 
magnetisation) are :— 
(13) For the wire before stretching. 10,000 ergs. 
(14) For the same wire after stretching . . . . 16,400 ,, 
The difference is considerable, but it is scarcely so striking as the other differences in 
magnetic behaviour which have been described in § 31 above. 
Heating effect of cyclic changes of Magnetism. 
§ 34. The energy expended in a cyclic process of magnetisation can take no other 
form than that of heat, diffused throughout the substance of the metal. Experiments 
have been made by Joule and others to determine by direct observation the heating 
effect of magnetisation in iron.* In most direct measurements of this quantity no dis¬ 
tinction is made between the heating effect due to the induction of electric currents 
and that due to changes of magnetisation per se, and so excellent an authority as Pro¬ 
fessor PtOWLAND, writing in 1881, has expressed himself as doubtful wdiether changes 
of magnetisation, considered apart from the currents they induce, give rise to any 
development of heat at all.t But no direct calorimetric measurements are needed to 
show that a cyclic process of magnetisation does give rise to a certain development of 
heat, even if it be performed so slowly as to make the dissipation of energy through 
* Joule, Phil. Mag., xxiii., 1843; Grove, Phil. Mag., xxxv., 1849; Villari, ‘ Nnovo Cimento,’ 1870; 
Cazin, Ann. de Chim. et de Phys., 1875; Trowbridge, Proc. Amer. Acad, of Arts and Science, 1879. 
Other references are given by Warburg in the paper cited in the text. 
t “ I do not mean to here affirm that no heat can be due to the demagnetisation alone, but that we 
have at present no experimental proof of such direct transformation. All the experiments hitherto 
made have merely given ns the heating due to induced currents.”— Rowland, ‘ Scientific American, 
Supplement,’ Nov. 5, 1881. 
