So 



NATURE 



[JV^ARCH i8v, i9(',9 



MAGNETIC RAYS. 



THE complex phenomena that occur at the kathode of 

 a vacuum, tube in the presence of a magnetic field 

 ^lave given rise to numerous TesBarchcs since the time of 



l^. B-i 



riuclcer, and a great number of the effects observed still 

 await an explanation. 



Birkeland has shown that if a cylindrical discharge tube 

 is placed in a longitudinal mag- 

 .netic field of gradually increasing 

 :strength, there is at a certain 

 value of the field an abrupt fall 

 in the potential at the terminals. 

 Almy has connected this with a 

 sudden change in the appearance 

 of the discharge. Willows has 

 found that a transverse field 

 •causes the discharge to pass 

 more readily below a certain 

 •value of the gas pressure ; 

 according to Peck, this effect is 

 not found if the kathode fall of 

 potential is greatly reduced by 

 the use of a hot lime kathode. 

 Broca has discovered that, in 

 addition to the kathode rays 

 which go in helices round tlie 

 lines of force, there is produced 



a second species which follow the lines. Villard calls these 

 magneto-kathode rays, and has shown that they are de- 

 flected bv an electrostatic field, but in a direction perpen- 



clusive, because the conducting power of the surrounding 

 gas may be sufficient to disperse any charge so collected. 



In a recent paper by Righi (_Accad. d. Set., Bologna, 

 May, 1908) the hypothesis is made that these rays are 

 electrically neutral doublets, more or less unstable, con- 

 sisting of an electron and a positive ion rotating round 

 each other. On account of its larger mass, the latter may 

 be looked upon as stationary while the electron moves in 

 an orbit around it. If the plane of rotation is perpen- 

 dicular to the field, the force acting on the electron, arising 

 from this field, will be radial, and the doublet may have 

 stability conferred upon it. If the plane of rotation is 

 inclined to the field, the electron, and hence also the 

 doublet, will tend to move up or down the lines of force. 

 Such doublets he calls magnetic rays, because the mag- 

 netic field is necessary to their stability. Fig. i shows 

 one forrri of tube used iSy Righi in his efforts to demonstrate 

 the existence of such rays. A is the anode, C the kathode, 

 R an electromagnet ; the source of current is a Holtr 

 machine. Figs. 2 and 3 show the appearance of the tube 

 when the discharge is passing, while the magnet is off 

 and on respectively. The part BE (Fig. i), according to 

 the author, consists of the rays in question ; to the left 



of E the field is too weak to confer stability on the 

 doublets. The luminosity, EF, behaves, to a magnet, like 

 the positive column of the ordinary discharge, but witb 

 E as anode and F as kathode. Fig. 4 shows 

 the effect of a transverse magnetic field on 

 this column. In order to obtain the magnetic 

 rays the field has to exceed a certain value, 

 depending on the gas pressure, and when this 

 value is reached the discharge becomes inter- 

 mittent, as may be shown by a rotating 

 mirror or telephone. The period increases 

 with the field. The paper contains numerous 

 photographs and measurements of the dis- 

 charge under different conditions, and there is 

 no doubt of its interest and suggestiveness, 

 although it cannot be said to have demon- 

 strated the actual existence of the hypothetical 

 ravs. R. S. W. 



PRIZE .^iUBJECTS FOR 

 SCIENTIFIC RESEARCH. 



A T the annual meeting of the Soci^t^ 



■^ Batave de Philosophic exp^rimentale de 



Rotterdam, a series of forty-eight questions, 



or proposed researches, were put forward for 



the coming year. Amongst these are the 



following : an exact critical review of the 



present state of knowledge of the volcanoes 



and volcanic phenomena in the archipelago of 



the East Indies, and an explanation of the 



origin of these volcanoes based upon these 



data or upon the author's own researches; 



^"' 3- an experimental research on the cause 



■diadar to the latter. Bv directing them into the interior I of phosphorescence, especially in animals of a lower 



•of a Faradav cvlinder, lie has been unable to prove that order ; an experimental research on the electrical properties 



.they carry a' charge ; the experiment is, however, not con- | of some metallic alloys ; an exact determmation ot tne 



NO. 2055, VOL. 80] 





