NA TURE 



357 



THURSDAY, FEBRUARY 15, 1 1 



RFXENT RESEARCHES IN ELECTRICITY 

 AND MAGNETISM. 

 Notes on Recent Researches in Electricity and Magnetism, 

 intended as a sequel to Prof. Clerk Maxwell's Treatise 

 on Electricity and Magnetism. By J. J. Thomson, 

 M.A., F.R.S., &c., Professor of Experimental Physics 

 in the University of Cambridge. (Oxford : at the 

 Clarendon Press, 1893.) 



THE supplementary volume to Maxwell's "Electricity" 

 which it was announced the present occupant of 

 Maxwell's chair in the University of Cambridge had in 

 preparation, was looked forward to with keen interest by 

 all electricians. It was sure, of course, to be a work of 

 great scientific importance ; but it was awaited with all the 

 more impatience because certain promises and allusions 

 in the new edition of Maxwell's treatise, lately published 

 under Prof. Thomson's editorship, had led to pleasant an- 

 ticipations that the supplement would be more or less of 

 a commentary on the treatise, and would deal with some 

 of the outstanding difficulties of Maxwell's electromag- 

 netic theory. One promise in particular, made in the 

 notes on the Electricity, we looked forward to 

 seeing fulfilled in the supplementary volume, that of 

 further discussion of the Maxwellian stress in the electro- 

 magnetic field. It is just here that the greatest difficul- 

 ties of Maxwell's theory present themselves to some at 

 least, and that a commentary such as the author could 

 have written would have been particularly valuable. 



Any little disappointment which may be felt at first as 

 to the contents of the volume vanishes when their solid 

 scientific value becomes apparent, and it is felt that 

 perhaps the author has done the right thing after all by 

 preferring to give a full account of the great work which 

 has been done in recent years, in confirming and verify- 

 ing Maxwell's theory, and in answering the questions it 

 has suggested. 



The book opens with an account of Prof. J, J. Thom- 

 son's method of regarding electric and magnetic pheno- 

 mena as produced by the motion of Faraday tubes of 

 electric force. This method is fully explained, and ap- 

 plied to the discussion of various physical phenomena, 

 such as electrolysis, the action of a galvanic cell, and so 

 forth. These show, perhaps, to the best advantage the 

 power of the method, which certainly enables a mental 

 image of what goes on in such cases to be more easily 

 and clearly formed. 



These tubes, according to Faraday's idea, start from 

 positive and end on negative electricity, and the positive 

 and negative electricities at the extremities of a tube, 

 being merely the two aspects or surface manifestations 

 of the state of strain existing in the medium within the 

 tube, are complementary and equal. 



According to Prof. Thomson's specification, a tube is 

 either closed or terminated by atoms. When it has a 

 length of the same order of magnitude as the distance 

 between two atoms in a molecule, the atoms are in 

 chemical combination ; when the length is of a higher 

 i order of magnitude the atoms are chemically free. 

 NO. 1268, VOL. 49] 



These tubes move through the field when electrical 

 changes take place, and by their motion produce magnetic 

 force, which is proportional to the velocity of the tube 

 moving at the point considered, and at right angles to 

 the plane defined by the tube and the direction of motion. 

 When a tube as a whole reaches a conductor, it shrinks 

 to molecular dimensions. 



To account for a steady magnetic field, which occurs 

 without dielectric polarisation, and therefore apparently 

 without the presence of tubes in the field at all, it is sup- 

 posed that there are passing through a given small area 

 just as many positive as there are of negative tubes, 

 (that is, there is a distribution of oppositely directed tubes 

 throughout the field, such that there is nowhere any pre- 

 ponderance of one kind over the other), but that these 

 two sets of tubes are moving with equal velocities in 

 opposite directions, so that the magnetic forces which 

 they produce reinforce one another. 



A quantity which the author calls the polarisation of 

 the medium is defined, for any given direction at a given 

 point, by the excess of the number per unit of area of 

 positive over negative tubes passing through a small 

 plane surface drawn through the point at right angles to 

 the given direction. When the dielectric is not air, the 

 unit area is supposed taken in a narrow crevasse cut in the 

 medium with its walls at right angles to the given direc- 

 tion. Thus the dielectric polarisation is exactly analogous 

 to the magnetic induction in the magnetic field as ordin- 

 arily defined. 



The momentum of the tubes per unit volume of the 

 field at any point is a directed quantity which is normal 

 to the plane defined by the magnetic induction and the 

 polarisation, and its components are proportional to the 

 rates of transference of energy, according to Poynting's 

 theory, in the directions of the axes across unit area held 

 at right angles to each of them. 



When the electric intensity of the field is due solely to 

 the motion of the tubes, they move at right angles to 

 their own directions with the velocity of light. 



This theory gives a clear idea of why both polarisa- 

 tion and conduction currents and the motion of charged 

 bodies produce magnetic effects. Everything is due to 

 the motion of Faraday tubes, and in all three cases the 

 Faraday tubes are moved through the field. 



It is questionable whether this theory will ever suc- 

 cessfully compete in electromagnetic discussions with 

 the reciprocal method, that of the motion of tubes of 

 magnetic force. Both were given by Faraday, who 

 speaks most unmistakably in his " Experimental Re- 

 searches," in the language of the modern theory of the 

 induction of currents by the motion of tubes of magnetic 

 induction across conductors situated in the field. It is 

 well to have both, and their use will serve to emphasise 

 what is of very great importance, the reciprocal cha- 

 racter exhibited very strikingly in the modification of 

 Maxwell's electromagnetic equations, given by Heaviside 

 and Hertz, of the relations between the electric and the 

 magnetic forces. 



After these preliminary discussions comes an account of 

 the phenomena accompanying the passage of electricity 

 through gases. This reviews and coordinates to a con- 

 siderable extent the experimental researches of Crookes, 

 Spottiswoode and Moulton, Hittorf, and others in this 



R 



