When a diode thermionic tube, having cylindrical symmetry, is placed in a magnetic field parallel to its axis it is commonly called a magnetron. If there is a given potential difference between the anode and cathode of the tube, and if the magnetic field is steadily increased, a sharp and pronounced decrease of anode current occurs when the field reaches a certain value. It is easy to show that, if electrons leave without velocity from a cathode of radius b, they will just graze a concentric anode of radius a at potential V when the magnetic field H has the value given by

The increase in the angle of reflexion from a powdered crystal varies from (1 − n) tan θ to (1 − n) (tan θ + cot θ) as its absorption increases. In either case the true lattice spacing of a cubic crystal is obtained by increasing the extrapolated lattice spacing by a fraction 1 − n of itself. The divergence of the reflected beam by refraction is small in comparison with other causes of line width.

In a previous paper a new method, based on Kemmer's β-formalism, of calculating meson processes was given for the case in which the meson interacts with an electromagnetic field. This method is now extended to the nuclear interaction, so that the whole of the meson theory can be given either in tensor or in matrix form, the former being preferable when the wave aspect of the meson is important and the latter when the particle aspect is dominant.

As examples of the matrix method, derivations are given of the cross-sections for the nuclear scattering of mesons and for the production of mesons from nuclei by photons. It is pointed out that the usual non-relativistic theory of the nuclear interaction is inadequate even for very small velocities.

The statistical theory of isotropic turbulence, initiated by Taylor (3) and extended by de Kármán and Howarth (2), has proved of value in attacking problems associated with the decay of turbulence. In its application to such hydro-dynamical problems, the theory falls into two parts, a kinematical part and a dynamical part. The kinematical aspect consists in setting up correlations between velocity components, or their derivatives, at two arbitrary points in the fluid, and reducing the form of the tensor thus obtained in accordance with the severely restrictive assumption of isotropic turbulence; the success of de Kármán and Howarth's investigations is largely attributable to their improved treatment of this purely kinematical problem. The dynamical part then consists in applying the implications of the equations of continuity and motion to the functions defining the correlation tensors, in order to obtain information concerning their functional dependence on time and on the displacement between the two points for which the correlations are computed.

In a previous piece of work(1) numerous data were obtained on the dependence upon magnetizing field of the Benedicks e.m.f. in nickel wire. The work was restricted to a study of the effect of the initial increase of magnetizing field, no hysteresis loops being examined. The curves were most remarkable in that the change of e.m.f. depended upon the direction of the magnetizing field with respect to the temperature asymmetry. In the paper quoted, it was suggested that the curves should be resolved into two components, symmetrical and anti-symmetrical with respect to the field. In the present paper the former component is termed the normal component, because, as is shown later, a simple explanation of it can be given at once. We call the antisymmetrical component the anomalous effect.

The results of the bombardment of mercury, lead and thallium by 9 M.e.V. deuterons are reported. The following radioactive isotopes have been detected: 5·5 min., 48 min., 36 hr., 60 day mercury isotopes; 4·4 min., 10·5 hr., 44 hr., and 13 day thallium isotopes; 10·25 min., 2·75 hr., and 54 hr. lead isotopes; 18 hr. and 6·35 day bismuth isotopes. The 10·25 min. lead isotope is positron emitting, an interesting result in an element of high atomic number. Absorption measurement have been made of the radiations emitted by many of these isotopes and assignments have been made in most cases.

In conclusion we wish to thank Dr N. Feather for valuable discussions, and also for making for us a Ra E source. We are indebted to Dr Lewis for advice in setting up the thyratron scale of eight counter. This paper would be incomplete without a sincere acknowledgement of our indebtedness to the hard work of past members of this laboratory who have been mainly responsible for setting up the cyclotron.

One of us (R. S. K.) is grateful to the Royal Commissioners for the Exhibition of 1851 for the grant of an overseas scholarship which made this work possible.

When γ-radiation passes through a gas it ionizes by means of fast electrons which produce clusters of secondary ionization at intervals along their paths. At high pressures a considerable amount of recombination of ions takes place in these clusters; in the present paper a theory is described which enables the proportion of ions escaping recombination to be calculated as a function of the gas pressure and collecting field. A review of the available experimental data concerning the variation of ionization current with pressure and collecting field is given, and it is shown that the predictions of the cluster recombination theory are in satisfactory agreement with these experimental data.

Jaffé has given a theory of initial recombination valid for a columnar distribution of ionization, and has shown that this theory is in agreement with experiments in which α-particles produce the ionization. A number of authors have applied Jaffé's equations to ionization produced by fast electrons regardless of the initial localization of the ions in clusters. It is shown in the present paper that such measure of agreement with experiment as is obtained by this procedure is only obtained at the expense of assigning incorrect values to certain known constants. Also in the case of X- and γ-rays the columnar theory predicts a variation of the proportion of recombination with the wave-length of the radiation which is much too rapid. It is further shown that the method based on Jaffé's equations which is used by Clay and his colleagues to extrapolate experimental ionization currents at finite collecting fields to saturation currents at infinite fields is liable to systematic error in a direction likely to lead to the deduction of a spurious wall effect or the exaggeration of an existing wall effect.

First-order calculations are given concerning the possibility of designing a β-particle spectrograph in which the correlation between the photoelectrons of the stronger “natural β-ray lines” and the disintegration particles of the “continuous spectrum” might be investigated. It has been shown that, with a coincidence counting circuit of resolving time 10−6 sec., and with a source of suitable strength, this investigation should be possible in the case of any natural β-ray line the absolute intensity of which is sensibly greater than 0.01. A successful investigation of this kind would provide important data for the analysis of a complex continuous spectrum into the “partial spectra” corresponding to different modes of transformation (and might furnish direct information regarding the internal conversion coefficients appropriate to the stronger γ-radiations involved).

The unusual physical properties of liquid He ii have led to considerable speculation concerning its internal structure. Since the entropy difference between the liquid and solid states tends to zero as the temperature approaches absolute zero, it appears that in the neighbourhood of zero temperature the liquid must have some kind of ordered structure, and London's theoretical investigations(1) indicated that the best agreement could be obtained with the observed energy and molecular volume by supposing the liquid to have a diamond lattice. On the basis of this work Fröhlich(2) then suggested that in the ordered state He ii possessed a diamond lattice, and that the disordering process consisted in an interchange of the atoms between this lattice and the vacant points of the body-centred cubic lattice from which the diamond lattice may be considered as derived.

A further study has been made of the deuteron bombardment of silver. The following radioactive isotopes are formed: 2·4 min., 26 min. and 225 day along with the silver fraction; and 6·7 hr. and 1 year along with the cadmium fraction. Results of absorption measurements of the radiations emitted are given. The energy-yield curves have been determined for the 2·4 min. and the 26 min. Ag isotopes and also for the 6·7 hr. and 1 year Cd isotopes. The formation of the 26 min. body from silver has been explained on the basis of a new type of disintegration, namely the (d-p, 2n) reaction.

The following correction has to be made to my recent paper on the adsorption of dipoles. Equation (10) and the sentence immediately preceding it are to be replaced by the following paragraph:

In the polynomial expressions for f1(x) and f2(x) the coefficients of the powers of x depend on η2 and η3 which are functions of T.