5i6 



NA TURE 



[October 28, 1909 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

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

 tnanuscripls intended for this or any other part o/ Nature. 

 No notice is taken oj anonymous communications.] 



Magnetic Storms. 



I.\ his article upon tlie recent magnetic storm (N.vture, 

 September 30) Ur. Cliree writes : — " Another difficulty 

 ill regarding the phenomena of magnetic storms as entirely 

 and directly due to the action of electrical currents 

 associated with aurora is that it is a frequent occurrence — 

 as on the present occasion — for the horizontal force to be 

 considerably depressed below the normal value when the 

 storm has apparently ceased and for some considerable 

 time thereafter. It is possible, of course, that the external 

 currents have partly demagnetised the earth, or at least 

 modified its distribution of magnetism, and that there are 

 recuperative tendencies tending to cause reversion of what 

 is for the time being a more stable distribution, but if 

 this be the true explanation the demagnetising action and 

 the recuperative tendencies are presumably in action during 

 the course of the storm, and profoundly modify the 

 magnetic phenomena." 



I wish to add to Dr. Chree's words the remark that 

 this phenomenon of decrease in horizontal intensity is 

 nothing but part of the phenomena I described a long 

 time ago,' and about which I exchanged opinions with 

 Dr. Chree in Terrestrial Magnetism. 



Later on I found occasion in several publications again 

 to deal with those phenomena of " Post-turbation " : also 

 recently", at the meeting of the Helvetic Association of 

 Natural Science at Lausanne. 



In a short statement I directed attention to the fact 

 that the results of my investigations on magnetic perturba- 

 tion are in accordance with the splendid results and theories 

 of Birkeland and St0rmer. 



-My results were shortly as follows : — 



A force of disturbance is always present ; it strongly 

 increases when a storm appears, decreasing afterwards. 

 The horizontal component of this force is chiefly directed 

 to the magnetic south, i.e. along the meridian of the 

 regular magnetism of the globe, thus lying in the plane 

 through the magnetic axis and the station. 



The vertical component, on the contrary, mostly has the 

 same direction as that of permanent magnetism, i.e. down- 

 ward in the northern hemisphere. Its character and sign 

 are much less constant than those of the horizontal com- 

 ponent. 



The regular post-turbation is most frequent at lower 

 latitudes. 



An extensive study of all the storms recorded at Batavia 

 has taught me that the post-turbation often emerges in 

 a negative sense (northerly), but shortly after turns and 

 reaches its maximum positive value. Afterwards it de- 

 creases at a slower rate, and this decrease continues until 

 a new storm (it may be a very small one) appears. I 

 found that successive quiet days show that same decrease. 

 This is the same that Dr. Chree also detected, and called 

 non-cyclic variation. 



At Lausanne I pointed to the fact that this kind of 

 disturbance corresponds to Birkeland's class of positive 

 equatorial disturbances. He also found negative disturb- 

 ances, but from my statistics it is evident that they are 

 much less frequent than the positive. 



As to the cause of these positive equatorial disturb- 

 ances, Birkeland, according to his experiments with his 

 terella and the theoretical calculations of Stormer, accepts 

 currents of electrons flying around the magnetic equator 

 of the earth. As for the negative ones, he accepts electrons 

 which move also in that plane, but through a loop in 

 their orbit, thus having a contrary direction. 



If we accept the cause of the post-turbation to be the 

 presence of an electric current, this current must be extra- 

 terrestrial, because the vertical component generally 

 increases when the horizontal one decreases. 



The less regular character of the vertical component may 

 be explained by the currents of induction raised inside the 



1 "Die magnetische N.ichstorung," Met^orclogische Zeiischri/t, 1895. 

 NO. 2087, VOL. 81] 



globe, .'\ccordingly, an effect of demagnetisation of the 

 earth seems to be improbable. 



Krom the inspection of thousands of magnetic curves 

 recorded at Utrecht, Batavia, and other non-Arctic stations, 

 I received tlie impression that the increase and decrease of 

 the post-turbation are fairly regular, the rapid oscillations 

 being superposed on this variation. 



No doubt these rapid oscillations are caused by currents 

 of electrons nearly approaching the earth, and this will 

 happen more frequently in the polar regions than in the 

 equatorial ones. Thus the action of the ring current will 

 show itself more distinctly at stations at lower latitudes, 

 the ring being nearer to them than to stations at higher 

 latitudes, and it will be less disturbed by other currents 

 coming very near to the earth. 



Dr. Chree concludes his article with wise words, say- 

 ing : — " To many minds subscription to some theory may 

 be a necessity for intellectual comfort, but in the case of 

 magnetic storms reservation of judgment appears at present 

 the more scientific attitude." 



I fully agree with him in this respect, but I think he, 

 who himself has contributed so much to the science of 

 terrestrial magnetism, will agree with the call for more, 

 activity. The work of Birkeland and StDrmer is splendid 

 indeed, but it is only in the power of international coopera- 

 tion, such as in the year 1S82-3, to unravel all the 

 problems of magnetic disturbance. 



We should repeat that work, considering that at present 

 we are able to do so much better and more completely than 

 our predecessors were in those days. 



Concentration of our efforts on special problems, I think, 

 would be more effective than the unsystematic accumula- 

 tion of material nowadays. 



The crowding of permanent magnetic observatories irf 

 Europe may be favourable to the solution of minor 

 problems ; it is a hindrance to that of the fundamental 

 ones, because it absorbs too great a part of the powers at 

 our disposal. W. v.\N Bemmelen. 



The Hague, October i";. 



Homogeneous Corpuscular Radiation. 



When a metal plate is subjected to a beam of Rontgen 

 rays, a corpuscular radiation is in general emitted, in 

 addition to the secondary radiation of the Rontgen type. 



This corpuscular radiation has been investigated by 

 various experimenters. They have shown that the intensity 

 and the absorbability of this radiation vary when different 

 metals are used, and that they are also dependent upon 

 the degree of " hardness " of the exciting radiation. In 

 particular, Cooksey has recently shown that the corpuscular 

 radiation excited by a " hard " primary beam is homo- 

 geneous, while that excited by a " soft " primary is hetero- 

 geneous. 



But the primarv beams used in these investigations were 

 necessarily heterogeneous, and it is therefore impossible to 

 decide with certainty which components were chiefly con- 

 cerned in producing the phenomena under investigation. 



It has been shown in various papers by Prof. Barkla and 

 myself that a series of secondary Rontgen radiations can be 

 obtained from the group of metals the atomic weights of 

 which lie between those of chromium and silver, each radia- 

 tion being homogeneous and having a perfectly definite 

 coefficient of absorption b}' a given metal, e.g. aluminium. 

 The absorption coefficients of the beams from the different 

 members of the series vary greatly in value ; thus for the 

 secondary radiation from iron the absorption coefficient by 

 aluminium is 240, while for that from silver it is only 67. 



1 wish to place on record a summarv of the results of 

 some investigations I have made upon the corpuscular 

 radiations excited in various metals when these homo- 

 geneous secondary beams are employed as primaries instead 

 of the heterogeneous primary beams used by previous 

 investigators. 



(i) It had been shown by Prof. Barkla and myself that 

 the penetrating power of the incident primary radiation 

 must exceed that of the homogeneous secondary Rontgen 

 radiation characteristic of a metal before the latter is ex- 

 cited. Using homogeneous beams, I have shown that when 

 the primary beam is only just more penetrating than the 

 secondary Rontgen radiation characteristic of the metal, 

 the intensity of both the secondary Rontgen radiation and 



