384 SECTIONAL TRANSACTIONS.— A. 



Dr. H. E. Ives. — The rate of a moving atomic clock (12.30). 



According to the theory of Larmor and Lorentz, a moving clock should 

 assume a slower rate than a stationary one. It was pointed out by Einstein 

 in 1907 that the newly discovered Doppler effect in canal-rays offered a 

 means of testing this prediction, but this test has been commonly considered 

 as beyond experimental practicability. This objection has recently been 

 removed, owing to the development by Dempster of a new design of canal- 

 ray tube. 



The present investigation is an experimental test of the Larmor-Lorentz 

 prediction, using these tubes. The hydrogen line 4861 A.U. has been used, 

 observations being made by means of a plane grating of 15,000 lines to the 

 inch, made by Professor R. W. Wood. The method of observation gave 

 on one plate, the lines due to motion in opposite senses. The experiment 

 gave a positive result, showing shifts which are independent of the orienta- 

 tion of the apparatus, and which agree, within the limits of experimental 

 error, with the theoretical values. 



On the assumption of a stagnant ether, this experiment, with that of 

 Kennedy and Thorndyke, establishes the reality of the Larmor-Lorentz 

 variation of clock rate, and the Fitzgerald contraction. 



(concurrently with above papers.) 

 Symposium on Magnetic alloys and X-ray structure (11.20). 



Prof. W. L. Bragg, O.B.E., Y.R.S.— Introduction. 



The magnetic properties of materials used for technical purposes have 

 been improved in the most remarkable way in recent years. Better alloys 

 have been discovered with a high permeability for small magnetising forces, 

 or with low hysteresis loss when subjected to magnetic cycles, or with a 

 high coercive force when used for permanent magnets. At the same time, 

 the theory of magnetism has made rapid advances in the hands of the 

 theoretical physicists, and X-ray methods of determining the atomic arrange- 

 ment in these materials have been improved. The present position is very 

 interesting because it is to be hoped that theory may now begin to play a 

 part in technical achievement. 



In this introduction to the discussion a brief survey is made of the mag- 

 netic properties of materials, and suggestions put forward as to the lines 

 along which improvement may be expected. 



Dr. E. C. Stoner, F.R.S. — The general theory of ferromagnetism 

 (11.40). 



In the Weiss treatment, which provides a qualitatively satisfactory formal 

 correlation of many properties of ferromagnetics, it is postulated that the 

 elementary magnets are acted on by a molecular field, equivalent in effect to 

 a magnetic field proportional to the intensity of magnetisation. Such a 

 field will give rise to spontaneous magnetisation below a critical tempera- 

 ture, the Curie temperature, and to paramagnetic behaviour above it. 

 The spontaneous magnetisation normally extends unidirectionally over only 

 small regions, domains, and the effect of an external field is to align the 

 directions of magnetisation of the domains. 



The elementary magnets in ferromagnetic metals are electron spins, and 

 the molecular field has been satisfactorily interpreted as arising from 

 quantum mechanical interchange interaction. The fundamental problems 



