TERRESTRIAL MAGNETISM.—MURPHY. 3038 
3. A wire permanently 3. A bar  peimanently 
twisted and then partly un- magnetized and then partly 
twisted loses or gains twist demagnetized loses or gains 
when jarred according as the magnetization according as 
untwisting is small or great. the demagnetization is small 
or great.” 
Mechanical strain produced by magnetization. In 1842, 
Joule found that a bar of soft iron lengthened when it was tetn- 
porarily magnetized in the longitudinal direction. (These effects, 
such as shocks, jarring and vibration in aiding the action of 
inductive magnetic force, were known to Gilbert.) When the 
magnetizing force was removed the bar shortened, but in general 
not quite to the original length. (This last sentence seems almost 
incredible to an Engineer, to take as granted that the magnetic 
force should be such as to strain the bar beyond its elastic limit.) 
Then again he says: “The actual elongation of an iron bar mag- 
netized to saturation was found to be from jy tO some Of its 
whole length. The extension varied approximately as the square 
of the intensity of magnetization (temporary or permanent). 
The general character was the same in soft or hard bars, but the 
effects were smaller with hard bars.” The results of Joule have 
been verified by Buff, Tyndal!, Mayer, and others. 
The effect of extreme cold was, according to Trowbridge, to 
diminish the moment of a steel magnet (magnetized at 20° C.) 
by about 60 per cent. 
The temporary magnetism of bars of cast iron, smithy iron, 
soft iron, soft steel, and hard steel magnetized by the earth’s 
vertical force, was found by Scoresby to be insensible at a white 
heat, but to be much greater at a dark red heat than at the 
temperature of the air. 
There are many facts that go to prove that the time any 
magnetizing force takes to develop the maximum magnetization 
that is capable of being produced is very small. The most 
wonderful evidence on this head is the fact that the telephone, 
which depends essentially on varying magnetic action, can repro- 
duce the sounds of human speech even to the consonants, 
