OCTOBEE 14, 1921] 



SCIENCE 



345 



I. Induction Effects. 



1. Eelation between field strength and mag- 



netic induction, permeability, suscepti- 

 bility, coercive force, retentivity, hyster- 

 esis, etc. 



2. Dia-, para- and ferromagnetism. 



3. Terrestrial magnetism. 



4. Alternating currents. 



, 5. Inductive eifects as influenced by tem- 



perature, mechanic strains, aging, etc. 

 6. Relation between susceptibility and chem- 

 ical properties. 

 II. Mechanical Effects. 



(a) Reaction effects between magnetic fields. 



1. Attraction and repulsion of mag- 



netic poles. 



2. Motion of electric conductors, sol- 



ids, liquids and gases, carrying 

 currents when placed in a mag- 

 netic field. 



3. Hall effect and its reciprocal rela- 



tions. 

 (6) Magnetostrictive Effects. 



1. Joule effect. Its reciprocal rela- 



tions. 



2. Villari effect. 



3. Wiedemann effect. Its reciprocal 



relations. 



4. Volume change. Its reciprocal 



relations. 



5. Change in resistance due to a 



magnetic field. 



6. Production of sound. 



7. Piezo- and pyromagnetism. 



8. Magne erystallic action. 



9. Effect of magnetic field on thermo- 



electric phenomena. 

 III. Magneto-optical Effects. 



1. Faraday effect. 



2. Kerr effect. 



3. Zeeman effect. 



4. Magnetic double refraction. 



Naturally one might question some points 

 in this classiiication. Certainly changes 

 would be made if we knew more about the 

 subject. Whatever the arrangement of sub- 

 jects a complete magnetic theory must ex- 

 plain all of the above phenomena. This is a 

 real task. In particular, the present magnetic 

 theories sidestep those phenomena listed 

 above as magnetostrictive effects, which as 



the outline indicates is about half of the 

 various magnetic effects. If the rotation of 

 the elementary magnets due to an external 

 magnetic field explains ferromagnetism then 

 one may properly ask if the rotation of the 

 elementary magnets might not also explain 

 the magnetostrictive effects because these 

 effects appear in ferromagnetic substances. 

 Poynting and Thomson^*' have called atten- 

 tion to the fact that these magnetostrictive 

 effects are yet to be explained on the molecular 

 hypothesis. They state. 



It would obviously require some further as- 

 sumptions as to molecular grouping or as to 

 molecular dimensions in different directions. 



The latter assumption has been a suggestive 

 one and some progress has been made along 

 this line, many of the magnetostrictive effects 

 may be explained as being due to the orienta- 

 tion of elementary magnets whose dimensions 

 vary in different directions. The work of 

 Barnett,^^ Einstein^" and deHaas and J. Q. 

 Stewart-^ favors the idea of an orientation of 

 the elementary magnet. Indeed our evidence 

 seems very strong that rotation of the elemen- 

 tary magnets due to an external field is a 

 part at least of all ferromagnetic phenomena. 

 The brilliant and highly significant work 

 of the two Comptons^' and their co-laborers^* 

 on the problem of the ultimate magnetic 

 particle has a very important bearing on this 

 phase of our discussion. Their interpreta- 

 tion thus far seems to argue against any- 

 thing turning due to an external field unless 

 it be something inside of the_ atom. If it is 

 something inside of the atom it would seem 

 difficult to explain the Heusler alloys or that 

 bulk iron is ferromagnetic; while ferrous 

 sulphate is paramagnetic and potassium fer- 



18 Poynting and Thomson, "Elec. and Mag.," p. 

 201, 1914. 



laBarnett, Fhys. Bev., 6, 240, 1915. 



20 Einstein and deHaas, Verh. d. deutsch. Fhys. 

 Ges., 17, 152, 1915. 



21 Stewart, Phys. Bev., 11, 100, 1918. 



22 Compton and Trousdale, Phys. Bev., 5, 315, 

 1915. 



23 Compton and Eognley, Phys. Bev., 16, 464, 

 1920. 



