Table of contents

The cathode spot of transient arcs using the liquid sodium-potassium alloy for cathode has been studied by means of the Kerr cell apparatus capable of taking a sequence of photographs. It is found that for a given current the spot takes, as in the case of mercury, the form of a line or broken line of total length roughly proportional to the current. This is also true for a spot in a magnetic field. Again as for mercury, if the current rises slowly the length of line increases proportionally to the current If the rate of growth of current is greater than a value lying approximately between 10⁸ and 2 × 10⁸ amp/sec., then fresh spots form and spread out radially from the point of formation into thin semicircular or circular lines moving with a maximum radial velocity of about 10⁴ cm/sec. The apparent current density of emission lies between 2.5 and 5 × 10⁶ amp/cm², and consequently is higher than for mercury.

The field and lens constants are calculated for two typical magnetic lenses as used in electron microscopes, using the relaxation method developed by Southwell for the solution of potential problems The results are compared with those based on simple analytic approximations to the field distribution in the work of Glaser and Ramberg, and are shown to agree closely as regards those characteristics on which the discussion of the performance of the lens is based

The fluxmeter consists of a conducting system of small capacity which is alternately exposed to and screened from the electric field by the movement of a rapidly rotating metal shield, thus producing an alternating E M F. of period 0.83 millisecond The output, after amplification, is displayed on the screen of a cathode-ray oscillograph and the record carries an automatic indication of the sense of the field every 7.5 milliseconds

At the maximum sensitivity employed the device gives a deflection of one centimetre in a field of 20 volts/metre, with a background noise-level of 0.15 cm. (3 volts/metre).

The response-time of the instrument is one half-cycle (0 42 millisecond).

The fluxmeter has been developed to give the rapid response and high sensitivity required for studies of the electric fields of thunderstorms in the intervals between the separate strokes of a lightning discharge.

A theoretical study is presented of the TM-waves that can travel along a slipping stream of electrons. A slipping stream is defined as one in which the electrons move in parallel paths with velocity which varies with distance transverse to the motion. It is found that amplifying waves can travel along a slipping stream for all frequencies. It is also found that the slipping-stream tube has the remarkable property of combining the characteristics of a two-beam tube and a travelling-wave tube This is due to the occurrence in the stream under suitable conditions of resonance layers, which act as highly reactive impedance sheets and can guide TM-waves of slow phase velocity in the same way as the spiral or corugated surface in a travelling-wave tube.

The slipping-stream tube behaves differently according to the fractional velocity range of the electrons. If the velocity varies linearly from V_-a to V_a across the stream and α = (V_a - V_-a)/(V_a + V_-a) < 0.42 it behaves primarily as a two-beam tube with plasma resonance frequency ω₀/2, and therefore having a cut-off frequency ω₁ = ω₀/α√2 and maximum gain 2.1ω₀/V₀ decibels per unit length, where ω₀ is the plasma resonance frequency of the slipping stream; in addition it has a low gain for ω > ω₁. However, if α > 0.42 it behaves primarily like a travelling-wave tube and the rate of gain is about 0.53ω₀/V_-a decibels per unit length at all frequencies above the plasma resonance frequency. This rate of gain is low compared with the gain possible with a travelling-wave tube in which the electron velocity is V_-a but it is achieved without the need for any external slow-wave waveguide.

Wave propagation along a slipping stream inside a waveguide, which in the absence of the electrons can guide a TM-wave of slow phase velocity, is also discussed. It is shown that this slipping stream travelling-wave tube has very much the same characteristics as a travelling-wave tube with a uniform electron beam Maximum gain occurs when the phase velocity v₀ of the wave in the empty guide is about equal to the velocity of the electrons nearest to the reactive impedance sheet of the guide - in the example considered this is the fastest electron velocity V_a A parametric set of curves is given relating the complex propagation constant to the frequency for different ratios v₀/V_a.

A new theory of ionospheric layer formation is developed, in which the temperature is assumed to vary linearly with height The equations are compared at each step with those obtained by Chapman in his theory of layer formation in an isothermal atmosphere. The equations for the maximum of electron density and its height are also given. The effect of the parameters on the shape of the layer is shown in graphical form./

The equations are somewhat complex in form, but an ingenious graphical method has been devised suitable for the application of the theory to results given in the form of a Booker and Seaton parabolic distribution of electronic density with height. By application of the theory to mean hourly values for four summer months in South Africa, values are obtained for the temperature gradient, the temperature at 200 km, and its variation over the middle part of the day. The results obtained are in accordance with previous estimates and offer numerical confirmation of the theory that the atmosphere expands bodily upwards during the middle part of a summer day

White light is passed through a Michelson interferometer in one of whose arms a thin lamina of the substance in the shape of a tension member is supported by a loading frame. The emergent beam, examined by a spectrometer, shows a continuous spectrum crossed by interference fringes. For the unstressed lamina the position of the fringes gives the variation of refractive index with wavelength For the stressed lamina the behaviour of the fringes gives the stress optical coefficients. No modification of the interferometer, and only three sets of observations on the fringes, are required. For the calculation of absolute values of the refractive index the values of two wavelengths must be known.

Full details of the method are given, and photographs of tests on Columbia Resin 39 are included. These show that CR39 becomes positively birefringent under tension

The systematic observations which have been made on the normal vibrations of free elliptical plates confirm Chladni's results The nodal systems may be divided into the same four classes of symmetry as those of rectangular plates.

The correspondence which exists between the vibrations of elliptical, rectangular and circular plates is traced Combined modes of vibration do not occur on elliptical plates as they occasionally do on rectangular plates