MAGNETISATION OF IRON AND STEEL. 641 



them in a long coil traversed by a current which was slowly raised 

 to a fixed value, and which was then made to decrease in the same 

 way, and the permanent magnetisation was determined. The bar 

 was then re-annealed at successive temperatures for the time specified 

 in the following table, then magnetised afresh in the same manner 

 and in the same direction. 



at a red heat (soft steel)... ... ... 157 



The curves of Fig. 250 represent the magnetic moment for unit 

 weight, or the specific moment that is to say, the quotient of the 

 permanent magnetisation by the density which corresponds to the 

 different degrees of reheating or of resistance, for bars of the same 

 diameter, 0.15 cm., and lengths respectively equal to 10, 20, 30, 40, 

 or 50 times the diameter. 



It will be seen that the reheating begins by increasing the specific 

 moment up to a maximum which is obtained for a lower temperature 

 of annealing, the longer is the bar. The moment diminishes then 

 very rapidly, and again falls to a very low value for steel, which is 

 heated to redness, or quite soft. With long bars, tempered at the 

 blue, a specific moment may be obtained, exceeding 100 G.C.S. 

 units in other words, an intensity of magnetisation equal to 780, 

 thus reaching about half the maximum magnetisation which soft iron 

 acquires. 



The amount of the magnetic moment is not the only quality to 

 be considered in a magnet ; this moment must be also as constant 

 as possible, and undergo no permanent alteration from changes of 

 temperature, shocks, vibrations, and time. Steel may acquire a 



VOL. II. T T 



