20 



NATURE 



[June 2^, 192 1 



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 of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



The Constitution of Nickel. 



Mv latest experiments have enabled me to obtain 

 the mass spectrum of the element nickel by using 

 the vapour of nickel carbonyl mixed with carbon 

 dioxide. The ordinary discharge tube was employed 

 to produce the positive rays, and difficulties of main- 

 taining a steady discharge were overcome to some 

 degree by the use of comparatively high pressure and 

 a heavy current. The rays were analysed in the 

 usual way by means of the mass spectrograph. 



The spectrum consists of two lines, the stronger at 

 58 and the weaker at 60. They are most conveniently 

 placed between the mercury groups of the third and 

 fourth order, with which they can be compared with 

 an accuracy of i/ioth per cent. The results were 

 also checked by comparison with the CO^ line 44, 

 and appear to be integral within the above error. 

 Nickel therefore consists of at least two isotopes. 

 The intensities of the lines are about in the ratio 

 2 : I, and this agrees with the accepted atomic weight 

 5868, F. W. Aston. 



Cavendish Laboratory, Cambridge, June 10. 



A Novel Magneto-Optical Effect. 



Early in April last, while my son, Malcolm 

 Thomson, was op>erating, in a buildinj^ of the River 

 Works plant of the General Electric Co., a resistance 

 welder for closing the seams of steel Langmuir mer- 

 cury vacuum pumps, in which work the current is 

 applied and cut off at about one-half second intervals, 

 there was noticed by one of the working^ force, Mr. 

 Davis, who happened to be favourably located, a 

 peculiar intermittent illumination of the space near 

 the welder as the current went on and off. My son 

 at once placed himself in a similar position and saw 

 the novel effect, and noted a number of conditions 

 accompanying it, perhaps the most important being: 

 that a single-turn loop from the welding- transformer 

 to the work and back was carrying about 7000 

 amperes, and that the luminous effect was spread in 

 the space in which would be located the magnetic field 

 from this loop; that the sunlig^ht was entering the 

 building^ through high windows and shining across 

 the space in which the field was produced at intervals ; 

 that the effect was most conspicuous when one looked 

 towards the shadows and across the sunbeams, and 

 also across the maijnetic field. 



This would be expressed by saying- that the best 

 effect was observed when the line of vision was down- 

 ward at an ang-le intersecting: the entering sunbeams, 

 and into the shadows under the beam furnished for- 

 tunately by a partition a few feet high, over which 

 the sunlii^ht came. The magnetic field, neg^lecting the 

 curvature of the lines, was, generally speaking, at 

 rig-ht anpfles to the line of sight and to the direction 

 of the sunlisjht. My son also noticed that the effect 

 of increased luminosity was coincident with the 

 putting on of the current, and disappeared at once on 

 cutting off the field. It was thus clear that it de- 

 pended on the establishment of the magnetic field. 

 He reported these facts to me, and they were con- 

 firmed' by me. Other observers were soon enlisted, 

 and on several favourable sunny days all the above 



NO. 2695, VOL. 107] 



observations were confirmed bv them. Further, my 

 son had not been able to see any effect when looking 

 across the sunbeam from the opposite side. This 

 means that, with the sunbeams streaming in from 

 the south, the effect was observecl looking southward 

 and downward, the windows admitting the light being 

 to the south. Looking from the south across the 

 beam gave no result, though it was mot possible to 

 look directly across the beam on a slant upward into 

 any dark shadows and at the same time have the 

 hne of vision cross the magnetic field. 



It is interesting to note at this point that the 

 lummosity filled the whole space, and extended as 

 far away as four feet or more from the magnetic loop, 

 and that it was not especially noted as more intense- 

 near the loop than at a distance therefrom of, say, 

 two feet or more. 



Mr. Malcolm Thomson had further observed that 

 by cutting out the loop from the secondary terminals 

 (clamps) of the welding transformer, and simply join- 

 ting those terminals by an iron bar, as is done' in re- 

 sistance welding, the luminous effect in the neighbour- 

 hood of the transformer was still visible, but was 

 much more feeble than when the heavy loop was used. 

 It occurred to me to examine the light by a large 

 Nicol's prism. It was found that there was" a distinct 

 polarisation of the light from the space. This means 

 that when the magnetic field was on the sunlight 

 was scattered in the direction of the observer from 

 the space occupied by the sunlight beam and the mag. 

 netic field, and that such scattered or deflected light 

 was polarised. 



It occurred to me, as a possible factor in the case, 

 that as the building was used in part to carr\- on 

 arc welding by iron arcs there might be suspended in 

 the air of the building iron particles or finely divided 

 oxides or compounds of iron which in some way were 

 oriented by the magnetic field, resulting- in the scat- 

 teretl light noted. This was confirmed in part bv 

 making the test observations when the large doors 

 of the building had been open for some hours. The 

 effect was present, though difficult to detect. This 

 led to the suggestion to bring an Iron arc into opera- 

 tion near the space in which the luminous effect had 

 been seen. This was done, and with an enhancement 

 of the effect. 



-At this stage the further observations were carried 

 on in the Thomson Laboratory at Lynn, Mass., with 

 the aid of the laboratory staff (A. L. Ellis, H. L. 

 Watson, Dr. Hollnagel, and others). 



Two sets of test apparatus were prepared at mv 

 suggestion. One large welding transformer was 

 mounted In a special room, into which the sunbeams 

 could be received in the afternoon as the windows 

 faced south by west. The secondary terminals were 

 joined by a large loop of heavy copper cable 

 (about 12 sq. cm. section) of a loop diameter of 

 06 m. The loop consisted of two turns. The 

 plane of the loop was vertical and was nearly north 

 and south, or In a plane parallel to the direction of the 

 entering sunbeams, so that the magnetic field would 

 be In the main horizontal and transverse to the light 

 of the sun entering downward as before. An Iron arc 

 was arranged to be operated so that the smoke from 

 It would rise from below and enter the field of the 

 loop, and by changlner the relative position of the 

 arc the smoke column, widening as It rose, could 

 be made to bathe the turns of the coil, cross its axis, 

 or, at a distance away, merely enter the field. .As the 

 experiments thus far had always Involved connection 

 to the shop plant, with 60-cycle alternating current, a 

 check apparatus was set up, consisting of a storage, 

 battery (of a type such as Is used in automobile 



