56 
SIR WILLIAM THOMSON ON THE 
the earth’s magnetic force. The reversed effects of the “ ons” and “ offs,” observed after 
this change, were really augmentations and diminutions of magnetism induced by the 
earth’s vertical force, and were therefore the proper effects for soft iron when subject 
to a magnetizing force of less than the Villari critical value. Further experimental 
investigation is necessary to explain the greater amount of effect, the same in kind as 
those observed before the stretching by 28 lbs., which the wire showed after it had 
been stretched by this weight. 
199. The experiments indicated in my preliminary notice of June 10, 1875, were the 
commencement of an elaborate series of investigations by Mr. Andrew Gray and 
Mr. Thomas Gray, which have been continued with little intermission from that time 
until now, and which are still in progress, with the general object of investigating 
the effects of longitudinal and transverse stress upon the magnetization of different 
qualities of iron and steel, and of nickel and cobalt. A separate series of investiga¬ 
tions was made nearly two years ago by Mr. Donald Macfarlane on the effects of 
torsion on the magnetization of soft iron, bringing out some very remarkable results, 
also included in this paper (§§ 223-229, below). 
§§ 200-212. Investigation by the Ballistic* method, of the change of Magnetization pro¬ 
duced in a specimen of exceedingly soft Iron Wire, by the application and removal 
of pulling force. 
200. The wire used in these experiments was specially prepared for this investiga¬ 
tion* by Messrs. Richard Johnson and Nephew, Manchester. It was of No. 22 
Birmingham wire gauge; its weight per metre was 3’47 grammes, and its diameter 
was '075 of a centimetre. A steel pianoforte wire of the same gauge would bear about 
230 lbs. on and off, without experiencing in consequence any permanent change of 
quality. This iron wire was so soft that, after it was stretched by a scale-pan weigh¬ 
ing 1 lb., which thenceforth was kept always hanging on it, an additional weight of 
14 lbs. on and off gave a permanent elongation of '4 per cent., and 4 lbs. more gave it 
a further permanent elongation of 1 '6 per cent, (making in all 2 per cent, of permanent 
elongation produced by 18 lbs. on and off, the permanent weight of 1 lb. being always 
on). The weight of 18 lbs. was applied and removed several times on the 14th of 
May, without producing any more permanent elongation than the 1'6 per cent, which 
was observed after the first on and off. During three-quarters of a year after that 
day no weight of more than 14 lbs. (in addition to the permanent 1 lb.) was ever 
applied to the wire, and electro-magnetic experiments were made upon it from clay to 
day, with little intermission, with “ons” and “offs” of 14 lbs., but sometimes with 
7 lbs. During all this time the length of the wire (about 4 metres) remained sensibly 
constant for the same weight, and the wire experienced regular elastic elongations 
* Compare §§ 178, 179. Phil. Trans, for 1876, p. 693. 
