1885.] Magnetisation of Steel, Cast Iron, and Soft Iron. 379 



find that the magnetic moments of the bars are got by multiplying 

 ^the ordinates by 41*085378. Therefore, to get their magnetic 

 moments per gram, we have to multiply the ordinates of IX and X 

 by 0-3604, and those of XI by 0-3287. 



The magnetising force is evaluated by the equation — 



F=4t™C. . . (3.) 



where n is the number of turns of wire in the coil per centimetre 

 of its length, and C the current strength in c.g.s. units passing 

 through it. 



The abscissae of the curves are scale divisions of the galvanometer, 

 with its magnetometer at the platform division 32, or the readings at 

 other divisions reduced to those of this division. Thus the abscissas 

 H 



multiplied by--r — represent the currents in absolute measure. 

 10 X oZ 



The magnetising force F is therefore obtained from the curves by 



H H 



multiplying the abscissae by 4mn — — , or irn —-. In the first eight 



experiments, w=45, and H=0'16 ; thus we have 0-2828 as the factor 

 for reducing the abscissae of the curves I to VIII inclusive. The 

 abscissae of curves IX, X, and XI are reduced by the factor 0'224, 

 n being 37, and H 0'16. 



From a comparison of the carves Nos. I to IV inclusive, it will be 

 seen that the " charcoal iron " has the highest magnetisability, and 

 the "soft steel" the lowest, while that of the "soft Scotch iron" 

 approaches very near to the former. With regard to retentiveness, 

 the " charcoal iron " shows the least, and the " soft eteel " the 

 greatest. 



Passing on to curves V to VIII inclusive, we find that the general 

 effect of annealing the wires has been to lower their retentiveness, 

 and to raise their magnetisability for all forces. In the " soft Scotch 

 iron " wire little difference has been made ; but in the others the 

 effect is very marked, and is most noticeable in the " charcoal iron " 

 wire with respect to its magnetisability, and in the case of the " soft 

 steel" wire with respect to its retentiveness. Comparing the results 

 of these eight experiments then, we find that the specimen which has 

 the highest magnetisability, and at the same time the lowest reten- 

 tiveness, is the annealed "charcoal iron " (7). Coming next to this 

 specimen, in both respects we have the " charcoal iron " wire (3), and 

 the annealed "common wire" (6), between which there is little or 

 no difference. The " soft steel " wire is the lowest of all in respect of 

 magnetisability, and highest in retentiveness. Annealing it, however 

 (8), has had the effect of bringing it very close to the " common 

 wire" (2). 



As regards the bars, the second specimen of cast iron is greatly 



2 c 2 



