June 23, 1921] 



NATURE 



521 



starting) arranged on a stand. In circuit with it, and 

 under control of a switch, was a coil of about 02 ni. 

 diameter, and giving a field due to about 2500 ampere 

 turns when the switch was closed. This second 

 apparatus could be moved about, and was entirely in- 

 dependent of supply circuits or static disturbances 

 which might be present in them. 



The first tests were made with the transformer loop 

 (representing a field of 20,000 ampere turns), and were 

 very striking. The rising smoke from the small iron 

 arc, only moderately visible in the sunbeam, became 

 (iecidedly luminous when the field was put on. Each 

 closure of the current switch to the primarv of the 

 transformer was instantly followed bv the brilliant 

 smoke effect, and the effect instantly disappeared on 

 the opening'. A black background had been provided 

 in front of which the smoke rose. After the arc had 

 been running a few minutes only it was seen that the 

 air of the room was carrying sufficient of the smoke 

 particles to give the effect anywhere in the space 

 fovered by the magnetic field and the sunbeams, even 

 J J number of feet away from the coil. In this case 

 i.he appearance was as if in the air there were diffused 

 (Ome substance or material which became visible only 

 m the combined sunlight and magnetic field. That 

 in this case the luminous effect is not greater near 

 the coil loop than some feet away indicates that 

 orientation, or whatever causes the effect, is complete 

 even in a rather weak field. Thorough ventilation of 

 the room bv opening windows caused the effect to 

 fade out gradually by removal of the active particles. 



The experiments with direct-current coil and batterv 

 conclusively showed that the effect was present with 

 it as with alternating current, and incidentally estab- 

 lished the fact that the effect on the particles is inde- 

 pendent of the direction of magnetisation. It is 

 doubtful if high-frequency tests would allow us -to 

 discover whether the establishment of the effect re- 

 quires time. Probably not. Observations made 

 through the axis of the loop of two turns show a 

 minimum of effect, from which it may be inferred that 

 it is not present if the viewing is exactly along the 

 field-line direction. 



Polarisation. — Having obtained, as described in the 

 foregoing-, a controllable and relatively brilliant sourct 

 of the luminosity, tests with the Nicol's prism were 

 resumed. It was soon noted that the polarisation was 

 decided as controlled by the magnetic field. More- 

 over, the verv curious fact was discovered by me. that 

 the fumes from the iron arc were composite so far 

 as analysis by the polarising- prism was concerned. 

 The bluish-coloured smoke arisinj? gave but little 

 effect, but there was with it a yellowish-grey fume, 

 which was highly luminous in one position of viewinif 

 bv the prism, and invisible when the orism was at 

 right angles to that position. This indicates com- 

 plete polarisation when the field is on for the light 

 diffused from the particles in the yellowish-grey fumes. 

 This is an extraordinary effect for which no explana- 

 tion suggests itself, for the field lines are not 

 straiirht, but wrap themselves around the coil or loop 

 in curved directions, and the effect is apparently com- 

 plete even with the fumes rising in the space where 

 the lines are strongly curved. 



It remains to use a vertical beam of lip"ht and make 

 tests from opposite directions across the field, also to 

 use artificial light instead of sunlight. It would seem 

 possible to design a small demonstration apparatus 

 consisting of a coil to be put on a batterv or lighting 

 circuit, A.C. or D.C.. a small iron arc between two 

 wires, a box with darkened interior to be filled with 

 fumes, havinp- two sides of glass, one for the admi" 

 sion of the lig^ht beam and the other a window c^t 



NO. 2695, VOL. 107] 



right angfles for observation. Two coils placed out- 

 side the box space and opposite each other, or capable 

 of application in different relations, would have 

 advantages. Eye shields to cut out extraneous 

 light and a tortuous chimney conveying the 

 smoke, but cutting off the light from the iron 

 arc, are desirable additions to the equipment, as also 

 an analyser as part of the apparatus for the polarisa- 

 tion effect. 



The Microscope. — Attempts have been made to catch 

 the particles in the smoke from the arc upon a glass 

 slide for microscopic examination as to their form 

 under high powers. That they are exceedingly fine is 

 evident from their remaining in suspension so long 

 in the air and diffusing themselves rapidly through 

 the air. That an exceedingly small amount of 

 material suffices for making the whole air of a large 

 room capable of showing the effect is evident also. 

 The sunbeam may enter the room, and its course is 

 not disclosed by them unless the magnetic field exists. 

 It seems natural to supf>ose that the particles consist 

 of some form of iron or iron oxide, but without proof 

 this cannot be fully decided. Other particles might 

 exist, giving such an effect, but it must be confessed 

 this does not seem probable. Other fumes and smoke 

 from arcs so far have given no results. The smoke 

 from a nickel arc does not give the effect. Whether 

 a cobalt arc will yield fumes behaving like iron 

 smoke is not yet known. 



The fumes and smoke of an iron arc were caught 

 on a clean microscope slide until a patch of sediment 

 of a slightly yellowish-brown tint, but very pale, was 

 deposited. Under moderate powers verv little of any 

 definiteness is shown, but under the high power of an 

 oil-immersion lens of about ih mm. focal length 

 there is disclosed a curious structure of particles 

 seemingly between 00002 and ooooi mm. diameter, 

 which particles are frequentlv strung together, 4, 5, 

 6, or more, in a line, giving the effect of a short piece 

 of chain made of small roundish particles, slightly 

 spaced apart, or of a short section of a string of 

 beads (round beads) not touching one another. Many 

 of these structures appear to be straight, and some 

 are curved. Evidently in a magnetic field these 

 chains of particles, presumably of oxide of iron and 

 magnetic, would line up and reflect or diffuse light 

 of the sun striking them. If the direction of vision 

 was such as to favour polarisation of the rays in a 

 direction nearly at right angles to thte incidence of the 

 solar beam the polariscope effect would be accounted 

 for measurably. Apart from polarisation, the lining 

 up of the chains would also account for the extra 

 visibilitv of the smoke under the conditions of the 

 experiment. 



It would seem from the foregoing that a consider- 

 able length of column of smoke frcMii the iron arc, 

 subjected transversely to a magnetic field, might be 

 expected to act as a means of obtaining polarised 

 light in the direction of the beam itself. This assumes 

 that there will be a considerable scattering of light 

 polarised as above described in a direction sidewise, 

 leaving the light which passes through polarised in a 

 plane at right angles. The apparatus might be com- 

 pared in its action to a Nicol's prism, transmitting 

 ravs in one plane and throwing out laterally those 

 in the other. This suggestion will be tested as soon 

 as proper arrangements can be made. 



The polarised light which is sent out from the 

 smoke particles in a direction transverse to the sun- 

 light beams, when the magnetic field is put on. is 

 in the same plane as that reflected from a sheet of 

 glass at the polarising angle receiving the same beam. 

 This fact is in accordance with what might be ex- 



