Magnetisation in the Length of Metal Rods. 



129 



of experiments would be brought into perfect harmony with those 

 already described, if it could be shown that the critical value of the 

 magnetising force was lowered when the rod was stretched. There 

 were reasons arising from the nature of my apparatus why I could 

 not attempt to prove this directly ; but an indirect method affords 

 strong evidence that this is the case. By the method of deflection 

 already described, it could be easily ascertained whether the position 

 of the "turning point" was affected by stretching a wire. Now, in 

 every case which I have hitherto investigated, it was found that the 

 critical value of the magnetising force was very approximately equal 

 to twice the magnetising force at the turning point. If therefore it 

 should appear that the position of the turning point was affected by 

 stretching, the presumption would be strong that the critical value 

 would be altered to a corresponding extent. 



Four deflection experiments were therefore made. In the first the 

 wire, which was of annealed iron 0*77 mm. in diameter, was supported 

 in a horizontal position inside the coil by means of corks ; gradually 

 increasing currents were passed through the coil and the galvano- 

 meter deflections noted as in former cases. The wire was. then 

 removed, and after being demagnetised was replaced inside the coil, 

 and a weight of 7 lbs. was attached to it by means of a cord passing 

 over a pulley. Once more the deflections accompanying increasing 

 currents were noted. 



The third and fourth experiments were similar to the first two, 

 except for the fact that the wire was magnetised to saturation before 

 any observations were made. The deflections recorded in the first 

 and second experiments are due therefore to the sum of the permanent 



Table V. 





Magnetometer deflections for 



Magnetometer deflections for 





total magnetisation. 



temporary magnetisation. 



Currents. 













Unstretched. 



Stretched. 



Unstretched. 



Stretched. 



0-012 



12 



16 



4-5 



6-5 



0-015 



16 -5 



24 



6 



8 



023 



25 



35 



10 



15 



0-027 



31 



40 



13 



18 



0-035 



38 



47 



18 



23 



0-042 



44 



51 



21-5 



27 



0-051 



51 



56 



27 



30 5 



061 



56 



59 



31 



34 



0-073 



62 



64 



36 



37 



0-108 



71 



70 



44 



415 



0-185 



80 



76 



52 



47 5 



0-274 



88 -5 



79 



55 



50 



VOL. XL. ■ K 



