CONTEMPORARY ADVANCES IN PHYSICS 329 



than 80 per cent of nickel so affects it that the magnetostriction is 

 increased — the tension seems as it were to have undone something, 

 which the magnetic field must restore before proceeding to the con- 

 traction which it operates upon the unstrained metal. 



Effect of Tension upon Magnetization 



The effects of strain upon magnetization are very complicated, and 

 one would almost despair of ever being able to interpret them, were 

 there not certain relations between them and the effects known 

 collectively as "magnetostriction" — between, to take the simplest 

 instance, the influence of magnetizing upon length and the influence 

 of lengthening upon magnetization — which indicate that law and order 

 reign even in this seemingly chaotic field. 



I mention the simplest instance only. A nickel wire, as we have 

 seen, shortens when magnetized parallel to its length; well, when 

 such a wire is shortened by compression, it becomes more magnetizable, 

 the value of / and the value of IjH produced by a continually-applied 

 field H increase; when it is lengthened by stretching (a much easier, 

 consequently a much oftener performed process!), its susceptibility 

 falls off greatly. An iron wire is lengthened when magnetized a 

 little, shortened when magnetized strongly; when it is lengthened 

 by stretching, the magnetization which a weak field imparts to it is 

 increased, that which a strong field imparts to it is diminished; the 

 magnetization-vs. -field curves for different extensions intersect one 

 another somewhere upon the "second segment." Again, a cobalt 

 wire, when lengthened by stretching, has a lower susceptibility in 

 weak fields and a higher susceptibility in strong fields than it does 

 when untensed; this corresponds to the rule governing the magneto- 

 striction of cobalt. To the Wiedemann effect there correspond a 

 magnetization which occurs when a wire carrying a current is twisted, 

 and a rush of current which occurs on twisting a wire already 

 magnetized. The signs of these effects, and of various others, vary 

 from one ferromagnetic metal to another, and vary in iron and cobalt 

 when the magnetization is sufficiently varied, in the ways which may 

 be deduced from the corresponding magnetostrictive effects. 



The variations in magnetization produced by extension may be 

 very much more striking than the variations in length produced by 

 magnetization. In nickel, for instance, the susceptibility of a wire 

 may be reduced to a tenth of its pristine value by stretching the wire, 

 although the utmost change in length which can be brought about 

 by magnetization is less than one part in ten thousand. 



