Certain Chrome and Tungsten Steels. 741 



specimen, a long narrow magnet being generally much less 

 affected than a short thick bar. 



Professor Ascoli of Rome has suggested a method by 

 -p. o> 9 which the intrinsic residual magnetism 



can be obtained from the hysteresis 

 curve, the dimension ratio of the magnet 

 being known. If OA be the residual 

 magnetism, and OB the coercive force, 

 then, if lines OP &c. be drawn from 

 such that tanAOP represents the de- 

 magnetizing coefficient of the specimen, 

 which depends for its value on the 

 dimension ratio, the ordinate MP represents the intrinsic 

 residual magnetism. As the bar diminishes in length, the 

 demagnetizing coefficient increases, and therefore the value 

 of MP becomes smaller. 



As the specimens examined were all cylindrical rods about 

 20 cm. in length and 0*9 cm. in diameter, the variation of 

 permanence of magnetism with dimension ratio does not 

 enter into the investigation, and the comparison instituted is 

 simply between the behaviour, under similar conditions, of 

 specimens similar in form, but varying in composition. 



In testing the specimens for permanence of magnetism, 

 the effect of repeated application of mechanical shock was 

 first observed. Each specimen, after its intrinsic residual 

 magnetism had been observed, was dropped, end-on, from a 

 height of 1 metre on to a wooden slab. To prevent the 

 specimen being differently affected in its two directions, it 

 was dropped alternately on either end, and tested magneto- 

 metrically after 1, 2, 6, 12, 20, 30, and 40 falls, or until its 

 magnetism reached a constant value, unaffected by further 

 falls. The specimen was then placed inside a steam coil 

 which was fitted inside the magnetizing solenoid, so that the 

 specimen could be heated to 100° 0. while in position for 

 testing. It was then alternately heated to 100° C and 

 allowed to cool to room temperature again, magnetometer 

 readings being taken at both temperatures. The specimen 

 at first lost a greater amount of magnetism at each heating 

 than it regained at the subsequent cooling, but the difference 

 between the loss and the gain became smaller as the process 

 was repeated, till a steady state was reached in which the loss 

 of magnetism on heating was exactly balanced by the gain 

 due to cooling. The specimen was then laid away and left 

 for about a month, when its magnetism was tested once more, 

 and the processes of application of shock, and heating and 



