IRON, STEEL, AND NICKEL TUBES IN THE MAGNETIC FIELD. 



551 



elongation (Z) transverse to the lines of magnetisation were calculated. These were 

 sometimes positive, sometimes negative, and were always small. Hence the dilatation 

 was given approximately by the quantity k + 2l. I reproduce from Mr Bidwell's paper 

 the following table in which the dilatations corresponding to various fields are given for 

 two iron rings, annealed and hardened: — 



Mr Bidwell's Results fob Iron Rings. 



Magnetising force, 

 in e.g. s. units. 



Increments and decrements of volume 

 in ten-millionths. 



King I. (annealed). 



Ring II. (hardened). 



10 



-27 



-11 



20 



-42 



-20 



30 



-47 



-20 



40 



-51 



-17 



60 



-51 



- 9 



80 



-48 



- 2 



100 



-46 



+ 3 



140 



-42 



+ 11 



180 



-39 



+ 17 



220 



-37 



+ 22 



260 



-35 



+ 26 



300 



- 32-5 



+ 30 



400 



-30 



+ 40 



500 



-29 



+ 44 



In both cases there is a maximum negative dilatation occurring in fields 50 and 25 

 respectively. With Eing II. the dilatation changes to positive in Field 85. Do these 

 results in magnitude and general character at all resemble the results of the present 

 paper ? 



My results must first be reduced to dilatations, and in making the reduction I 

 assume, in accordance with the statement at the end of last paragraph, an approximate 

 uniformity of behaviour all along the tube. Were the strain simply isotropic, then the 

 dilatation would be measured by the ratio of the change of volume included within the 

 bore to the volume of the bore. Eeducing accordingly, we get the following tables of 

 dilatations for iron and nickel — the steel tubes because of their extraordinary diversity 

 of behaviour being disregarded. The dilatations are expressed in the same unit as in 

 Mr Bidwell's table above. 



It is evident at a glance that the dilatations for the iron tubes are of the same order 

 of quantity as those of Mr Bidwell's rings. Before any further comparison can be 

 instituted between his results and mine, it will be necessary to measure the longitudinal 

 elongations of the tubes directly. This I hope to carry out in the immediate future. 



On the whole, Mr Bidwell's values are greater than mine ; but this is not surprising 

 when we consider the essential difference in the method of experiment. Moreover, in 



VOL. XXXVIII. PART III. (NO. 13). 4 E 



