676 REPORT— 1894. 



11. The Volume Charu/es which accomjjany Magnetisation in Nickel Tubes. 

 By Professor C. G. Knott, D.Sc. 



The method of experiment was similar to that employed in the determination 

 of corresponding changes in iron and steel tubes, and already described in a former 

 communication.^ Three nickel tubes, cut originally from the same solid bar, were 

 turned and bored. They were of the same length (47 cm.) and the same external 

 diameters (4-2 cm.). The internal diameters were as follows: — No. I., 2-543 cm. ; 

 No. II., 1-586 cm. ; and No. III., 0-692 cm. 



The decrease of volume in the tube of widest bore (No. I.), when subjected to 

 a longitudinal field of 600, was so large that it had to be measured with the naked 

 eye. The liquid meniscus in the capillary tube moved outwards through a distance 

 of 3 cm., which corresponded to a volume change of 2-4 cubic millimetres. This, 

 with a total internal volume of 224-47 cubic centimetres, gives a dilatation of 

 fully -10-^ 



In the following table some of the more striking results are indicated. Under 

 each heading of field is a column containing the corresponding cubical dilatations 

 for the three tubes. 



Cubical Dilatations x 10'. 



Thus in lowest fields the cubical dilatation is negative in Tubes I. and II. Very 

 soon, however, it becomes positive, but changes back to nesrative in still higher 

 fields, and so continues to the highest fields used. In Tube III. there is no evidence 

 of the negative dilatation in low fields, so that with it there is only one change of 

 sign. The thicker the wall, the higher the field in which the change of sign takes 

 place. In moderate and high fields the changes of volume are distinctly greater 

 in nickel than in iron or steel. 



1 2. On Hysteresis in Iron and Steel in a Rotati'iig Magnetic Field. 

 By Francis G. Baily, M.A. 



It has long been surmised that the hysteresis in iron may have different values 

 according to the mode of variation of the direction of magnetisation. As a deduc- 

 tion from Professor Ewings molecular theory of magnetism Mr. James Swinburne 

 pointed out that the value of the hysteresis in an alternating field should be dif- 

 ferent, and obey a diHerent law, from that of iron in a rotating magnetic field, the 

 latter probably being the smaller, especially at a high induction. The point 

 has not, however, received any experimental verification until now. 



The experiments here described consisted in causing a powerful electromagnet 

 to revolve on an axis concentric with the bore of the pole pieces, which formed 

 parts of a cylinder. The magnetic field between the two pole pieces rotated with 

 the electromagnet producing it. In the polar cavity was placed a finely laminated 

 cylindrical armature of iron or steel carried by hollow centres in fixed supports, 

 and held by a spring attached to the armature spindle and to the fixed support 

 respectively. On rotating the field-magnet there is a force due to the hysteresis of 

 the armature tending to cause it to revolve with the magnet. This is prevented by 

 the spring, and the deflexion of the spring is a measure of the torque exerted on 

 the armature, w hich is proportional to the hysteresis in the armature per reversal, 

 and is independent of the speed of revolution of the magnet. The deflexion is 

 ol served by the movement of a beam of light reflected from a small mirror on the 



' See B.A. Itflj)orts, 1892, p. 659. 



