STRAIN ON THE PHYSICAL PROPERTIES OF MATTER. 



15 



magnetism for a given magnetic stress was measured by the difference between the 

 deflections which ensued when the battery circuit was closed for the first and for the 

 tenth times ; thus, if D l and D 10 be these two deflections respectively, the permanent 

 magnetism is assumed to be measured by D! D 10 , and the temporary magnetism 



b D 



10 . 



From this experiment it appears that until the resistance in the battery circuit has 

 become about 302 B.A. units there is no perceptible permanent magnetism. The 

 electromotive force of the GROVE'S cell was 1 '9 volts ; the number of turns per centi- 

 metre in the magnetising solenoid was ,-^j, and the total resistance in circuit was 

 302 X '9889 legal ohms. The magnetising stress in electromagnetic units would 



consequently be 



4ir x 1-9 x 10" x 70 



or 



302 x -9889 x 10 x 11'43 ' 



04896. 



The above magnetic stress is little more than one-fourth of that due to the 

 horizontal component of the earth's magnetic force. The magnetising solenoid was in 

 this experiment placed horizontally and at right angles to the magnetic meridian, so 

 that the iron was uninfluenced by the magnetism of the earth ; but other similar 

 experiments seemed to show that, in whatever position the iron was placed, the mole- 

 cules required a very appreciable magnetic stress to produce permanent molecular set.t 



[October 6th, 1887. It may be objected to the above experiment that the length 

 of the pieces of iron is not sufficiently great in comparison with their diameter ; J but 



* The resistance of the battery and solenoid amounted to 2 B.A. units, which together with 1000 

 B.A. units added from the box makes the number here given. 



f Later experiments, however, proved that even with the small magnetic stress quoted above a 

 permanent deflection is produced. (See the passage following.) 



J Cf. EWINO, ' Phil. Trans.,' 1885, pp. 533-536. 



