292 On the Magnetisation Limit of Iron. 



1 



.Number oi 

 experiment. 



.Length, ot iron 

 wire. 



Tractive force with 

 current = 40 

 amperes. 



Tractive force 

 per square inch.. 





. , 

 men. 







IDS. 



Ike 



IDS. 



X 



U 2 





OOD 



2 



0-4 



1-016 



397 



3 



0-6 



1 -059 



414 



4 



0-8 



1-074 



420 



5 



1-0 



1 -078 



422 



6 



1-2 



1-078 



422 



It will be seen from this table tliat the magnetisation of the first 

 specimen agrees well with the result of my former experiment with 

 similar charcoal-iron wire of the same length and0'05 inch diameter^ 

 the tractive force being proportional to the sectional areas of the 

 wares respectively. It will also be observed that the tractive force 

 increases with the length of the wire up to l'O inch, beyond which no 

 higher degree of magnetisation was obtained. 



That the magnetisation limit (422 lbs. per sq. inch, or 29 67 kilos, 

 per sq. cm.) was virtually arrived at in these experiments was 

 shown from the fact that, when the current transmitted round the 

 electro-magnet was reduced from 40 to 26 amperes, the amount of 

 the tractive force remained constant. 



That the single-pole method of determining the magnetisation 

 limit of magnetic substances compares favourably with the double- 

 pole method was proved by the following experiment : — 



The electro-magnet employed in the previous experiments con- 

 sisted of a single limb of the large magnet used in my demon- 

 stration of the indefinite increase of the magnetic and electric forces 

 from quantities indefinitely small.* The second limb of this magnet 

 was replaced, and both limbs were furnished with flat tapering 

 poles having a space of 0*3 inch between them. 



A piece of the same wire used in the previous experiments, 1-2 inch 

 long, was bent into a U-form, with the inside of the limbs 0*3 inch 

 apart, and carefully balanced over the tapered poles. It has been 

 shown that the same length of wire in the straight form required 

 a weiglit of 1'078 lbs. to detach it from the electro-magnet. It 

 might, therefore, have been expected that the U-formed iron wire 

 would have indicated at least double this amount, or 2*156 lbs!, 

 whereas the tractive force obtained was only l'G25 lbs. Some of 

 the diminished magnetic effect was no doubt caused by the induc- 

 tive action of the tapered poles upon each other, and also through 

 the mechanical force of separation not being applied so directly as 

 in the experiments with the straight iron wires. Enough, however, 



* 1 Roy. TJoc. Proc.,' April, 1866. ' Phil. Trans. ' 1867. 



