NA TURE 



[February 6, 1896 



LETTERS TO THE EDITOR. 



[7'he Editor does not hold himself responsible for opinions ex- 

 pressed by his correspottdents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications. ] 



Velocity of Propagation of Electrostatic Force. 



Dr. Bottomi.ey's note published in Nature of January 23, 

 quotes an extract from my Baltimore Lectures of October 1884, 

 in which this subject is spoken of, with an illustration consisting 

 of two metal spheres at a great distance asunder, having 

 periodically varying opposite electrifications maintained in them 

 by a wire connecting them through an alternate current dynamo. 



For an illustration absolutely freed from connecting wire and 

 all complications, consider four metal spheres. A, B, f, d, with 

 their centres all in one straight line ; — their relative magnitudes 

 and positions being such as shown in the accompanying diagram. 

 Let each of the four be initially electrified, A and c, positively, 

 B and d, negatively. Let the charges on c and d be so strong 

 that a spark is only just prevented irom passing between them 

 by the influence of B and A. Let A be gradually brought nearer 

 to B till a spark passes between them. Will the consequent 

 spark between c and d take place at the same instant or a little 

 later ? It is not easy to see how this question could be answered 

 experimentally ; but remembering the wonderful ingenuity shown 

 by Hertz in finding how to answer questions related to it, we 

 need not perhaps despair to see it also answered by experiment. 



The elastic solid theory restricted to the supposition of incom- 

 pressibility (which is expressed by Maxwell's formulas) makes 

 the difference of times between the two sparks infinitely small. 



The accompanying photograph may perhaps interest those 

 who are engaged in the photography of invisible objects. It was 

 taken in the following way. Upon a piece of board I placed a 

 sensitive plate, on this a penny-piece with the obverse side 

 downwards, and on the top of the penny-piece a ^'V-inch cedar 

 board. The whole was exposed to the light emitted by the 

 burning of sixteen inches of magnesium ribbon at a distance of 

 six inches. I developed with pyro-ammonia. An inspection of 

 the photograph will show a distinct image of the Queen's head. 



On repeating the experiment with fifteen inches of magnesium 

 ribbon at six inches, but without the use of the cedar board, the 

 part of the plate surrounding the coin was solarised, whilst the 

 part underneath was over-exposed, so that no trace of the image 

 was visible. 



I then repeated the last experiment, using a slow lantern- 

 plate, and burning four inches of ribbon at nine inches distance ; 

 on development a faint image of the Queen's head was visible. 

 Hence it is only a matter of exposure and development to 

 produce a much better result than the one presented. 



The phenomenon does not appear to be due to the varying 

 thickness of the coin, since the impression of the reverse side has 

 not modified the result, but rather to the different directions in 

 which the penetrating rays are refracted from the irregular 

 refracting surface on to the sensitive plate. W. Saunders. 



[A FAINT image of the Queen's head is quite visible upon the 

 print received from Mr. Saunders, but it will not bear reproduc- 

 tion. — Ed. Nature.] 



A STORY was current at Cambridge some forty years ago that 

 an aspirant to mathematical honours replied to the question. 



The unrestricted elastic solid theory gives for the difference of 

 times the amount calculated according to the velocity of the 

 condensational-rarefactional wave. 



But I feel that it is an abuse of words to speak of the " elastic 

 solid theory of electricity and magnetism " when no one hitherto 

 has shown how to find in an elastic solid anything analogous to 

 the attraction between rubbed sealing-wax and a little fragment 

 of paper ; or between a loadstone or steel magnet and a piece 

 of iron ; or between two wires conveying electric currents. 

 Elastic solid, however, we must have, or a definite' mechanical 

 analogue of it, for the undulatory theory of light and of magnetic 

 waves and of electric waves. And consideration of the definite 

 knowledge we have of the properties of a real elastic solid, which 

 we have learned from observation and experiment, aided by 

 mathematics, is exceedingly valuable in suggesting and guiding 

 ideas towards a general theory which shall include light (Old and 

 New), old and new knowledge of electricity, and the whole of 

 electro-magnetism. Kelvin. 



The New Actinic Rays. 



May I point out that an unnecessary amount of energy is being 

 expended on Rontgen's photographs — I mean electrical energy. 



I have succeeded in obtaining perfectly sharp and fully-exposed 

 negatives from an action of four minutes' duration, even when a 

 thin aluminium plate is placed in front of the sensitive film, and 

 the rays are excited in a Crookes' bulb connected direct (?>. 

 with no Leydens inserted) with the secondary terminals of an 

 Apps' induction coil, which gives (in its present condition) a 

 three-inch spark in air when worked, as in the present experi- 

 ments, by three small accumulator cells. This is much smaller, 

 however, than that used in the published experiments of others 

 who have been doing similar work. 



University College, London. . Alfred W. Porter. 



NO. 137 1, VOL. 53] 



©© 



"Construct a prism through which no ray can pass," in the 

 terms following : — 



"Take a prism of wood : then if no ray passes through, 

 what was required is done. But if a ray does pass thr ough, 

 paint it." 



Surely a marvellous anticipation of Rontgen's X-rays ! 



R. B. H. 



The Stress in Magnetised Iron. 



I AM glad that Dr. Chree, in his letter published in Nature 

 of January 23, has raised a discussion of this matter, regarding 

 which, as he says, the most contradictory statements are to be 

 found. For some time I have been aware that the passage 

 referred to in my book on "Magnetic Induction in Iron" 

 requires correction. The magnetic stress, B^/8ir, in a long rod 

 or ring uniformly magnetised, is there spoken of as if it were of 

 the same nature as a simple longitudinal stress of compression, 

 producing a contraction of the length in consequence of the 

 elasticity of the metal. Dr. Chree, if I understand him rightly, 

 would treat it as of the same nature as a simple longitudinal 

 stress of tension, producing an elongation of the iron. 



But it now seems clear to me that both of these views are 

 equally wrong. There is no proper comparison, in the general 

 state of magnetised iron, with the stress in a loaded pillar or the 

 stress in a stretched rope. 



Take the case of a uniformly magnetised ring, where we have 

 no complications due to end effects. Imagine a plane of section, 

 and call the halves of the ring A and B. According to the first 

 view, A is, as a consequence of the magnetisation of the ring, 

 pushing against B, and B against A. According to the other 

 view, A is pulling B, and B is pulling A. But if A is either 

 pushing or pulling B, the equilibrium of B demands that some 

 other force must act on it to balance this push or pull. No such 



