June 30, 192 1] 



NATURE 



553 



As 1 1-5 

 P 13-3 



S 8-30 



or 12-2 



I lO-I 



or 80 



Group V.A. 

 52 290 



Group VI. 



2-05 170 



205 or 250 



Group VILA. 



280 283 



280 or 224 



Inert Gases, 



2-36 

 or 2-52 



2-37 

 or 2-Z7 



250 



2-95 

 2-95 



27-2 

 or 290 



31-5 

 or 34-2 



208 

 or 304 



29-8 

 or 236 



He 



Ne 



A 12 



254 

 16 



130 



20-8 



246 



2-86 730 



267 428 



303 364 



In the first group of the periodic table the products 

 shown in each of the fourth and sixth columns of the 

 above table are fairly concordant, so that we may 

 conclude that the work done in the removal of an 

 exterior electron is nearly proportional inversely as the 

 radius. 



The same remark applies to four elements of the 

 second group, while the members of sub-group B 

 diverge considerably from the values for the A group. 

 According to Urbach (Phys. Zeit., February, 192 1, 

 p. 1 16), the elements of the B sub-group have a double 

 ring of electrons in the outer zone, while those of the 

 A sub-group have a single ring. In the case of the 

 inert gases, neon and argon, the diameters estimated 

 by Bragg give products in the fourth column in far 

 better accord with theory than those found from the 

 cube-root of the atomic volume set forth in the sixth 

 column. 



The values for certain elements in groups iii.-vii. 

 are given in the table for comparison, but our know- 

 ledge of ionising potentials is as yet too fragmentary 

 to permit of any definite conclusions. 



The ultimate solution of this problem may involve 

 calculations of the character given by Sir J. J. 

 Thomson in his recent paper in the Philosophical 

 Magazine (March, 192 1, p. 526). 



I am indebted to Prof. A. LI. Hughes for his 

 assistance in endeavouring to collect the most trust- 

 worthy values for the ionising potentials. 



A. S. Eve. 



Macdonald Physics Building, 



McGill University, Montreal, June 6. 



A Novel Magneto-Optical Effect. 



The interesting observation recorded by Prof. Elihu 

 Thomson in Nature of June 23, p. 520, seems likely 

 to have a bearing on the old Reichenbach experi- 

 ments, which were for the most part disbelieved by 

 orthodox science, but on which Sir William Barrett 

 and others made some careful observations, to ascer- 

 tain what truth there might be in them. The effects 

 could not be denied, but they were capricious ; and 

 in view of Prof. Elihu Thomson's discovery, it seems 

 possible that the luminosity may have been visible 

 to sensitive p>ercipients when there was a trace of 

 magnetic dust in the room and when other light was 

 not excluded. The obvious precaution of excluding 

 other light may have been the condition which mili- 

 tated against the examination of the phenomenon, 

 which it was then thought was presumably of a sub- 

 jective character. Oliver Lodge. 



June 25. 



NO. 2696, VOL. 107] 



Helicopters. 



One often sees published statements to the effect 

 that a helicopter has been invented and that wonderful 

 things are expected of it. If the design gets as far as 

 an actual trial a few alterations are found to be 

 required, and then nothing more is heard of the 

 matter. 



To make a machine which without an extravagant 

 expenditure of power will raise itself vertically and 

 remain poised in the air is possible and most desir- 

 able, and the many failures in the attempt to do this 

 are all attributable (omitting mistakes in mechanical 

 design) to the same cause, namely, that of giving an 

 insufficient area to the lifting surface. 



The sort of area required may be gathered from the 

 following illustration. Let two aeroplanes facing in 

 opposite directions be"' connected by a few hundred 

 yards of light line joining their wing-tips. The 

 machines so connected could rise and circle round 

 each other without much difficulty. When in the air 

 the line might be hauled in until the wing-tips were 

 almost in contact, and in this condition the combined 

 machines would form a helicopter. There would be 

 no banking, as the connecting line would take the 

 centrifugal force, but more power would be required 

 than when the machines were flying independently 

 on account of the lower speed and efficiency of the 

 inner pair of wings. 



The function of a screw or lifting surface is to 

 generate a downward current of air, the reaction of 

 which on the surface shall be equal to the weight 

 supported. If L^ is the cross-section area of this 

 current (dependent on, though not identical with, the 

 area of the lifting surface), V its velocity, and W the 

 weight, L^V^x (constant somewhat greater than half 

 the density of air) = W. Hence LV is a constant, and 

 V is inversely proportional to L. 



The power required to maintain the current is WV, 

 and can therefore be reduced by making L/V large. 



For instance, if W can be sustained on a current 

 of area L* by P horse-power, only half this power 

 would be required if the current area were 4L*. 



A. Mallock. 



A Physical interpretation of the Energy Quantum. 



The work of Bohr (Phil. Mag., 1913-15) indicates 

 that we may assume stability only for some electronic 

 orbits, i.e. amplitude changes occur discontinuously. 

 ^ye arrive at a similar conclusion in the case of the 

 vibrating atoms of solids if we accept the quantum 

 explanation of the change in their specific heats with 

 temperature. 



It is here suggested that the amplitude of a periodic 

 disturbance in the aether can alter only by definite 

 amounts which depend on its frequency, so that as 

 soon as any part of a wave-front meets with some- 

 thing that is capable of inducing a decrease in ampli- 

 tude, such as a suitably situated electron, that part 

 suffers a definite decrease of amplitude, which extends 

 back into the wave-train (uniformly in all directions 

 in an isotropic medium) to the extent of one quantum. 

 The aether is relieved of its energy of strain, not 

 continuously, but in quanta. 



With the modification suggested above the wave 

 theory renders understandable, on one hand, pheno- 

 mena such as interference, and, on the other, pheno- 

 mena such as the photo-electric effect, a detailed dis- 

 cussion of which is here impossible owing to lack of 

 space. 



More light might be thrown on this subject by a 

 mathematical treatment. Ian Aucken. 



The County School, Long Eaton, 

 June 13, 



