566 



NA TURE 



[October 13, 1898 



such as these, together with the systematic determination 

 of selected physical constants. With larger funds such 

 results could be obtained, and there is no reason to fear 

 that with a carefully chosen Committee, a good organ- 

 isation, and the best Director that can be secured, the 

 National Physical Laboratory would in due time take its 

 place among the great scientific institutions of Europe, 

 and would forge another link in the chain which binds 

 science and industry together. / 



EXPERIMENTAL PHYSICS, 

 Lehrbuch der Experimental-Physik. Von Eduard 

 Riecke. Zweiter Band, Magnetismus, Elektrizitat, 

 Warme. (Leipzig : Verlagvon Veit und Comp., 1896.) 



IN Nature for August 20, 1896, we reviewed the first 

 volume of this work, and there stated what seemed 

 to us to be its most notable features. The second 

 volume strikes us as being even better than the first : 

 the author, at any rate, seems to move in the subjects 

 here treated with still more grace and freedom. 



The treatment of the subjects is clear, and, so far as 

 we have seen, always accurate, though the methods 

 adopted are not always the newest. Perhaps, it may be 

 argued, they are none the worse for that. However, in 

 one or two places, there are described at some length 

 various pieces of apparatus which hardly deserve a place 

 in a modern book on electricity. An electrician may, for 

 example, know nothing of the " unit jar," and not be a 

 whit the worse. Yet Prof. Riecke gives "Ein voll- 

 standiges Bild von der Konstruktion der Massflasche " ! 



Dielectric action is illustrated by well-chosen and 

 instructive diagrams. The' theory described is one pre- 

 cisely analogous to that of magnetic induction and 

 magnetic force, in which the medium is supposed to be 

 made up of polarised molecules, the opposite charges of 

 which act at a distance like other electric charges ; 

 while the electric induction is defined as the electrostatic 

 force in a crevasse at right angles to the polarisation, and 

 the electric force as the electrostatic force in a cylindrical 

 hollow along the lines of polarisation. Thus we have 

 in electricity, as in magnetism, the equation 



S = g + 4^3. 



Here a distinction is drawn between the true and the 

 free distribution on the plate of a condenser, a mode of 

 discussing the external action of the condenser which is 

 supplemented by an all too short account of the Max- 

 wellian view of the subject. 



Prof. Riecke gives at p. 23 a simple construction for 

 finding the direction of a magnetic line of force at any 

 point P. Draw to the point a line CP from the centre C 

 of the magnet, and find a point Q such that CQ — \ CP. 

 Draw from Q a perpendicular QR to CP, meeting the 

 magnetic axis in R. RP is the direction of the line of 

 force at P. It ought to be stated in the text that this 

 construction, which is easily derivable from the polar 

 equation r = <; sin "^6 of the line of force, is only applicable 

 to the case of an infinitely short magnet ; that is, it can 

 only be applied for an ordinary bar magnet when the 

 distance CP is very great in comparison with the length 

 of the magnet. 



The subject of electromagnetism is fully dealt with so 

 NO. 151 I, VOL. 58] 



far as the magnetic action of a current element, and the 

 mutual force between two current elements are con- 

 cerned. The law of Laplace (which was also given by 

 Savary and by Ampere) that the magnetic force pro- 

 duced by a current y in an element C of a circuit of 

 length ds at a point P at distance r from the element 

 and making an angle 6 with CP is yds%\n 6 j r^, and acts 

 at right angles to the plane of the element and P, is first 

 stated and used for the ordinary applications. Then from 

 that, by the principle of action and reaction, is obtained the 

 electromagnetic force on a current element yds in a field 

 of intensity H, making an angle 6 with the element is 

 yH ds sin 6. It is not noticed here, however, that taking 

 the magnetic action of an element of current to be as 

 stated in Laplace's formula, the reaction must exist in 

 the same line as the action, and hence to get the electro- 

 magnetic force on each element the reaction must, after 

 the method of Poinsot, be reduced to a force on the 

 element and a couple. 



All these laws of action of elements however are, it 

 should be more emphasised, incapable of absolute 

 demonstration. It is impossible to experiment with 

 elements, and so settle the question, and no confirmation 

 obtained by arriving at the observed actions of complete 

 currents is proved in the least, inasmuch as the addition 

 to the action of an element of any term, which integrated 

 round the circuit gave a zero result would give another 

 law, equally valid so far as the evidence goes. The 

 same point requires mention again later when Ampere's 

 law of the mutual action of two currents is discussed. It 

 seems therefore to be demonstrably certain that in the 

 ordinary theory of circuits it is impossible to arrive at a 

 unique law of the mutual action of elements. Yet time 

 is still wasted on the search for it. 



Notwithstanding the narrow limits of the book as 

 compared with many other Lehrbiicher, Prof. Riecke has 

 succeeded in compressing an immense amount of valuable 

 matter into his chapters on electricity and magnetism. 

 Of course the pages are large and well filled, and there 

 is far more than would be contained in an English book 

 of the same number of pages, but the author has suc- 

 ceeded wonderfully in contriving to give an account in 

 so much detail of electro-optics, including the electro- 

 magnetic theory of light, and of dynamo-electric 

 machinery. 



The final chapter, Elektrochemie, Electrolyse, contains 

 a fair discussion of the motion of ions, of electrolytic 

 dissociation, winding up with a sketch of the energy 

 theory of the voltaic cell. 



The final part of the second volume deals with heat, 

 and here again, in 130 pages, the author effects quite a 

 marvel of condensation. Temperature, expansion, the 

 air thermometer, all are soundly and clearly treated, and 

 there is an absence of the terrible confusion about scales 

 of the mercury and air thermometers which is so 

 common. For example, we came across again the other 

 day the statement that air is an excellent thermometric 

 substance because its expansion is so uniform. The 

 same thing is generally claimed in the same books for 

 mercury, and the authors never seem to think that this 

 uniformity is not absolute, but must be relative to some 

 standard. They do not perceive that the standard they 

 set up is really the expansion of the mercury itself in 



