542 



PROFESSOR KNOTT ON THE STRAINS PRODUCED IN 



§ 12. Complete Cycles and Hysteresis. — The changes of volume which are the 

 subject of discussion in the last three sections, are the changes which occur at make or 

 break of the magnetising current, after the tube has been subjected to several applications 

 of the same current in alternating directions. While the necessary observations were 

 being made, the meniscus in the capillary was often seen going through a curious suc- 

 cession of movements before coming to its final position. As an example, by no means 

 the most extraordinary, take the case of Iron Tube No. III. Brass Cap. In Field 467 

 the diminution of volume was —'59 (l0~ 5 c.c); but at the instant of applying the 

 magnetising current the meniscus first moved along the capillary to a distance which 

 indicated an increase of volume 15 '3, and then rapidly moved back through its original 

 position to the final position of rest. At break of the current the meniscus, in a some- 

 what similar manner, reached the final position of zero change after a rapid excursion 

 to a point which indicated a decrease of volume 9'8. These rapid excursions of the 

 meniscus far beyond the limits of volume change finally indicated gradually diminished 

 in extent as the magnetising current was decreased in intensity. The following are the 

 observations for this particular tube, the behaviour of which may be taken as a fairly 

 average example. 



The numbers in the second and fourth columns headed " Excursion " give thel 

 changes of volume corresponding to the maximum displacements of the meniscus at the 

 instant of make and break of the current ; the third and fifth columns contain the 

 final volume changes at make and break, which have, of course, the same numerical 

 values. 



Iron Tube III. : Brass Cap. 



Field. 



At Make. 



At Break. 



Initial Excursion. 



Final Volume Change. 



Initial Excursion. 



Final Volume Change. 



467 



364 



288 



212 



137 



102 



82 



69 



66 



+ 15-3 

 + 13 

 + 10-8 

 + 8-2 

 + 5-5 

 + 3-9 

 + 2-9 

 + 2-3 

 + 1-3 



- -59 



+ -io 





 -1-30 

 -3-42 



- -52 

 + -52 

 + -67 

 + -62 



-9-8 

 -9-8 

 -9-8 

 -8-2 

 -1-6 

 -1-3 

 -1-0 

 -1-0 

 - -7 



+ -59 



- -10 





 + 1-30 

 + 3-42 

 + -52 



- -52 



- -67 



- -62 



The first thought that occurs as we study these numbers is, that the initial excursion 

 is due to the volume change passing through its successive stages as the current gradually 

 works up to its steady value. Both magnetisation and volume change lag behind the 

 magnetic field, which, because of self-induction, requires some measurable time to attain 

 its final strength. Now in low fields there is increase of volume. Hence, since a high 

 field in being established must necessarily pass through the lower values, it is quite to 



