ON THE MEASUREMENT OF MAGNETIC HYSTPIRESIS. 
85 
Influence of Permanent Set upon the Eflects of Cijcles of Torsion. 
§62. The experiments of §61 made on specimen (2) showed that np to twists 
giving a permanent set of about 13 revolutions in a length of 65’7 centims. both 
By and W become greater when the torsional stress is relieved, and that beyond the 
limit of 13 revolutions the reverse is the case. This observation led us to inquire 
into the forms taken by tbe two curves connecting By and W with a cyclical 
torsional couple applied to a piece of soft iron wire, which had been previously 
twisted so as to have a considerable jiermanent set. The actual specimen was that 
used in § 58 without any further annealing. In carrying out these experiments, tlie 
wheel was turned through a definite number of revolutions, measured from its 
position before the wire was strained, thereby giving the wire a j^ermanent set. The 
wheel was then set free so as to allow the wire to untwist itself as far as possible, 
and then about 10 cycles of twisting were given to the wire, the load producing the 
torsional couple varying between the limits of 300 grammes. After these cycles 
of twisting, a set of magnetic observations was taken as the next cycle of twisting 
was gone through. In every case the magnetic force was put through 20 cycles of 
reversal before the magnetic observations were made. 
One eftect of the cycles of twisting was to diminish the “ permanent set ” of the 
wire. Thus, reckoning from the position of the wheel when the wire was set free 
after a twist of 50 revolutions, 10 cycles with the load between tlie limits of 
± 300 grammes caused the wheel to turn back through f revolution. 
It will be noticed that the curves in figs. 13, 14 do not always form closed figures. 
This is probably iu the main due to the fact that the preliminary cycles of twisting 
were gone through in a few minutes, while tlie one during wliich the tests for 
By and W were made occupied aliout an hour. We liave often observed a similar 
efiect when a copper wire is put through cycles of loading and unloading. Tliere is in 
this case so much “ creeping” when a load is applied, that, if a cycle of loading and 
unloading with a given range of load is performed slowly after a numlier of cycles of 
loading and unloading performed comparatively quickly with the same range of load, 
the curve connectino; the elong^ation and the load is not closed. 
The curves exhibit in a higlily developed form the want of symmetry which was 
rudimentary in the experiments of § 58 (fig. 11). 
In every case the maximum magnetic force was kept at the constant value 
Hg = 5'0, so that the experiments might be comparalde with those of §61. The 
lengtli of the wire, 65‘5 centims., vras ?d.) 0 ut the same as in the experiments of § 61. 
The results are given in the following table and are exhil)ited in figs. 13, 14. 
The + sign prefixed to a load in the table indicates that the couple due to it has 
the same sign as tlie coujile which gave the wire its permanent set. In order to 
show all the curves on a single diagram we have taken a different position of the zero 
