RESEARCHES IN MAGNETISM. 
625 
The same results are shown in Plate 67, fig. 56, where three groups of curves are 
given. The lower group shows the induced magnetism, and the middle group the 
residual magnetism, in terms of «§. The upper group shows the values of the ratio of 
residual to induced magnetism, also in terms of .£). 
The crossing of the curves of 3 (induced) and <§ should be noted. It corresponds, 
in kind, to the behaviour of iron, and shows that in steel, as in iron, any increase of 
stress gives first increase and then decrease of susceptibility, according as the mag¬ 
netisation falls short of or exceeds a certain critical value. Each curve lies at first 
higher and afterwards lower than curves referring to smaller values of the load. 
The middle group of curves shows that the same remark applies to residual 
magnetism. 
Further, the curves which represent the ratio of residual to temporary magnetism 
in relation to cross each other in the same way. There is thus, for each load, a 
critical value of the magnetisation, below which the ratio of residual to temporary 
magnetism is greater, and above which it is less, when the wire is loaded than when it 
is unloaded (or loaded with a smaller load). For example, under a load of 20 kilos, 
the retentiveness is much greater than under a small load or no load, so long as the 
magnetisation is feeble, but less when the magnetisation is strong. 
The observations taken with no load confirm what has been already said (§ 44) as to 
the absence of retentiveness in steel, under weak magnetising forces. But it is very 
remarkable how the presence of stress increases the retentiveness under weak mag¬ 
netising forces. A force of 1 unit, for instance, leaves sensibly no residual magnetism 
in the wire when there is no load, but leaves a residue amounting to nearly 0*4 of the 
induced magnetism when there is a load of 20 kilos. 
§ 110. A piece of iron wire, which, like the last, had (after annealing) been stretched 
beyond its limit of elasticity, was next examined in the same way (June 14, 1882), 
the successive states of load being 0, 5, 10, and 14*8 kilos. The diameter of the 
wire (after being stretched) was 0*72 mm. and its length was 30'5 cm. The load 
employed to stretch it was 20 kilos. 
The induced and residual magnetism of this wire, found under these various states 
of stress by applying and removing a large number of progressively increased values 
of <§, are given in the table below, and the same results are shown in Plate 67, fig. 57, 
where the curves give the relation to <£) of 3 induced, 3 residual, and their ratio:— 
