Table of contents

The principles of the design and details of the construction of a single-stage glycerol vapour pump of 3 in. diameter are described. The pump, which is made of glass, has a maximum speed of 80 1./sec. The range of pressure over which it performs satisfactorily and its backing pressure requirements compare favourably with two- and three-stage pumps. A reason for the good performance of a single stage against high backing pressures is suggested.

By the manipulation of a single switch the instrument records on two telephone message registers (i) the number of groups of objects studied, (ii) the total number of objects in the groups. The switch is foot-operated so that the user's hands are free.

Some experiments are described in which a counter-spectrometer method is compared with an X-ray photographic method as a means of determining the integrated reflexion of a number of reflexions. The advantages and limitations of the photographic, ionization and counter-spectrometers are discussed, and it is concluded that for long exposures the photographic spectrometer is as sensitive as the counter-spectrometer used for a few minutes and more sensitive than the ionization spectrometer. For convenience and speed of measurement the counter-spectrometer is probably the best instrument.

The modified Glan-Foucault prism described has advantages over the normal type due to the reduction of multiple reflexions in the air film. The prism is quite economical in the use of calcite when cut from the original rhomb.

The insensitive period following a count in a Geiger counter is measured by two methods. Each relates only to the periods following fully recovered pulses. The first method is an extension of the oscillograph method of Trost (1) and of Ruark and Branmer (2). A pulse interval discriminator is described which triggers the oscilloscope only when the counter is in a fully recovered state. This method is accurate for measurements of the dead time of a counter associated with a given recording circuit. By the second method, the time of recovery to any preselected fraction of the full pulse amplitude is measured. This is done by means of an electronic gate which automatically sets its length equal to the recovery time. This period is displayed on a meter.

The instrument described, known as a `Vane', works on this principle: a system of blades or `vanes' at the end of a rod is inserted into the soil and the torque required to cause the soil to fail under the stresses exerted by the vanes is measured. For soils of high clay content there is a linear relationship between torque and compressive strength. A simple theory is given for the instrument accounting for torque in terms of cohesion and angle of internal friction. The theory is in good agreement with experiment for angles up to at least 10°. The effect on the torque of the rate of rotation of the vanes has been investigated, and it is shown that the behaviour of the Vane is analogous in some respects to that of a Couette viscometer.

A description is given of an experimental cathode assembly, designed to be fitted to the standard target head of a Crystallographic X-ray Unit, which makes it possible to obtain an adjustable focus 1 cm. in length and down to 1/2 mm. in height, without a surrounding halo. The stability of the tube current is discussed.

Following a brief description of the fundamental structure of ionization chambers, the limitations of the accuracy of radiation intensity measurements, due to the `soaking in' of charge into the insulator, are discussed, and a method of greatly reducing this effect by suitable design of the electrode assembly is described.

The use of thermistors (thermally sensitive resistors) for the measurement of small temperature differences is reviewed and some practical details of manipulation are presented. An automatic apparatus for recording the freezing-point and another for measuring the boiling-point of a liquid are described in which thermistors are employed as the temperature sensitive elements. Reproductions of some tracings of records so obtained are given. The apparatus discussed may be used for the estimation of chemical purity, for the determination of molecular weights, or for recording small pressure fluctuations in a system.

A tumbling test for compressed cylindrical pellets is described. The pellets are placed in a cylinder, of glass or mild steel, which is rotated about an axis perpendicular to its axis in a vertical plane. It is shown that for any length of tube there is a speed of rotation at which the loss in weight is a maximum, and this moves to lower speeds as the tube length increases. With a fairly short tube the curve of remaining weight against speed of rotation is very flat after the minimum is reached, so the test can be controlled by a revolution counter without accurate speed control. The behaviour of pellets pressed at various pressures was examined, and it was found the pelleting properties of different materials can be ranked by means of the test. The tumbling test, unlike compression or impact breaking, is free from the personal bias.

A technique is described for the precision measurement of crystal lattice parameters with a discussion of the effect on the ultimate result of all errors, those due to inaccurate measurement, penetration of the specimen and divergence of the incident beam. It is shown that the errors of the first two types are negligible and the correction for the third can be calculated. Figures are given for the lattice parameter of iron.

An apparatus for the measurement of the magnetic susceptibility of rocks in a field of 0.5 oersted is described, together with one for the determination of the direction and intensity of the residual magnetism.