4o8 PROCESSOR KNOTT ON THE STRAINS PRODUCED IN IRON, 



in diameter. Two new sets of iron tubes, distinguished as the A tubes and the B tubes, 

 were studied. The B tubes correspond in dimensions with the nickel B tubes ; the A 

 tubes are twice as long. Each original iron bar gave in succession seven tubes, the 

 internal diameters of which increased from two-eighths of an inch (No. I.) to fully an 

 inch (No. VII.). The internal diameters of the nickel tubes range from three-eighths 

 (No. II.) to six-eighths (No. V.) of an inch. A further boring was in this case out of 

 the question, since the material had begun to crack, and the tube consequently to leak. 

 The dimensions of these and the other new tubes are given in numerical detail in 

 Table I. at the end of the paper. 



Having thus briefly sketched the history of the research, I propose in what follows 

 (1) to discuss in full the results for the A and B tubes, and (2) to compare with these, 

 and elucidate by their means, the comparatively incomplete results obtained with the 

 large tubes, both old and new. 



The existing old tubes are No. VII., 3, 5, 7 of iron and of steel ; and No. VII., 1, 

 4, 7 of nickel. 



The iron and steel tubes, No. 9, are the thinnest walled of all the large tubes, and 

 differ from the others in having no internal ledge on which a washer could be screwed 

 down under the cap. The measurement of the volume changes in these thin tubes 

 required a different method of fitting the cap, and, indeed, a different cap altogether. 

 The results originally obtained with them were not regarded as altogether satisfactory, 

 and were accordingly omitted in Part I. They are given below, for the sake of com- 

 pleteness, along with the elongations. 



Excluding the eighteen temporary tubes which formed the successive stages of the 

 iron, steel, and nickel tubes, No. VII., we have in all forty tubes, whose changes of form 

 and volume in various magnetic fields are now to be discussed. 



§ 2. Methods of Experiment. — With each of the A and B tubes four distinct 

 experiments were made. These were : — 



(1) Measurements in various magnetic fields of the corresponding changes of 



volume of bore. 



(2) Measurements in the same fields of changes of length. 



(3) Measurements in the same fields of changes of volume of the material of the 



tube. 



(4) Measurements in the same fields of apparent external changes of volume, the 



tube being plugged and treated as a bar. 



The first form of experiment was conducted after the manner described in Part I., 

 $ 7. The metal tube and the connected capillary glass tube were filled with water, and 

 the changes of volume measured by the displacements of the end of the water column 

 in the capillary. 



In the second form of experiment the change of length was measured by means of 

 a lever and mirror arrangement, similar in essence to the arrangements used by other 

 experimenters (Joule, Barrett, Bidwell, etc.). The tube under investigation rested 





