[ 693 ] 
XXVI. Electrodynamic Qualities of Metals (continued from Phil. Trans. Vol. 146. Read 
Feb. 28, 1856). — Part VI. Effects of Stress on Magnetization. By Sir William 
Thomson, F.B.S. & c . 
Beceived and Bead May 27, 1875. 
178. In Parts III. and IV. of my first series of papers under this title (Transactions 
of the Royal Society for February 1856), I described experiments discovering effects of 
stress on the thermo-electric quality and the electric resistances of metals. About the 
time those experiments were made I also made several nugatory attempts to discover 
the effects of stress on magnetization ; and eighteen years have passed before I have 
been able to resume the investigation. Early in the year 1874 I made arrangements to 
experiment on the magnetization of iron and steel wires in two different ways — one by 
observing the deflections of a suspended magnetic needle produced by the magnetization 
to be tested, the other by observing the throw of a galvanometer-needle, due to the 
momentary current induced by each sudden change of magnetism. The second method, 
which for brevity I shall call the ballistic method, was invented by Weber, and has 
been used with excellent effect by Thalen, Roland, and others. It has great advantages 
in respect of convenience, and the ease with which accurate results may be obtained by 
it ; but it is not adapted to show slow changes of magnetism, and is therefore not fit for 
certain important parts of the investigation. On this account I am continuing arrange- 
ments for carrying out the first method, although hitherto I have obtained no good 
results by it. 
179. On the other hand, I have found the ballistic method very easy and perfectly satis- 
factory in every respect, except that it does not show the slow changes of magnetization. 
It was by it that all the results which I am now going to describe were obtained. The 
apparatus, which is very simple, is represented in the accompanying sketch (fig. 1). 
A A' is the wire whose magnetism is experimented on. In my first experiments it was 
a piece of steel pianoforte-wire, No. 22 *, Birmingham wire-gauge, that is, weighing about 
3‘54 grammes per metre, and therefore of -7644 of a millimetre in diameter. It is 
about 5 metres long, and its upper end is firmly fixed to a beam in the ceiling of the 
Physical Laboratory of the University of Glasgow, where all the experiments have been 
made. To the lower end is attached a pan bearing weights, by means of which 
different amounts of pull may be rapidly applied and removed from the wire when 
desired. Over a portion CO of this wire 28'7 centimetres long there is wrapped 
* This is the wire used in the American Navy and in British cable-ships for deep-sea soundings. Its 
strength to resist pull is such that it hears about 230 lbs. (104 kilogrammes), or the weight in air of 29 ’4 kilo- 
metres (or 15-9 nautical miles) of its own length. 
MDCCCLXXVI. 5 D 
