Table of contents

A method is described for estimating the particle size distribution (Stokes' diameters) of insoluble powders in the sub-sieve range. The powder is dispersed in a suitable liquid and the particles allowed to settle on to the pan of a torsion balance. The cumulative weight settled is read at chosen time intervals by direct deflexion of the torsion wire. An alternative method is to use the return twist of the torsion wire. The size distribution by weight within the particulate mass is deduced from the curve relating the cumulative weight with time, the procedure adopted for calculation being described. The method is simple and speedy and can be operated by relatively unskilled labour.

A description is given of an electronic device for measuring small transient displacements of magnitude down to 10 μin, the apparatus being suitable for use whenever it is possible for part of the equipment to be mounted on a stable support and a small, light-weight steel plate to be attached to the moving part. The apparatus was devised initially for measuring the relative and absolute movements of concrete road slabs under the wheel loads of vehicles. The device is based upon the variation of output of a high-frequency oscillator caused by the movement of a metal disk near its tuning coil, and the output is linear over the range up to 0.10 in. The equipment is used with a persistent cathode-ray tube for visual estimation of displacements, and with cathode-ray recording gear when permanent photographic records of the displacements are needed.

The construction of a very simple microscope hot stage for the study of optical interference figures at elevated temperatures is described. A crystal under examination remains fully accessible, and with a negligible loss of heat in operation cooling arrangements are unnecessary. Temperature may be measured by means of a thermocouple. An example of performance is given.

A simple experimental arrangement is described in which a plane replica grating is used to obtain high dispersion with an infra-red spectrometer in the 3 μ region. The pre-monochromator unit for the grating spectrometer is built into the normal spectrometer radiation unit and uses the rocksalt prism of the spectrometer.

A projection microscope, modified for measurement of the small angle-scattering of tracks observed in nuclear emulsions, is described. The apparatus has been used to determine the masses of cosmic-ray particles which come to rest in the emulsion. If single scatters are eliminated, a function of the scattering angle and the range may be found which is independent of energy and has a Gaussian distribution.

A simple static pressure probe is described which will give good accuracy in a fixed position in a natural wind of rapidly varying direction and speed. It is in the form of a long cylinder with two radial holes separated by a critical angular distance. The static pressure is obtained as the average of the pressure on the two holes, the average being formed by equal capillary resistances between each hole and the common tube to the manometer.

Two modifications of the cone and plate viscometer are described. The first, for measurements at low shearing stresses (10 to 100 dyn/cm²), employs a phosphor-bronze strip to apply torque to the cone. The second, a direct reading instrument for measurements at a relatively high stress (4 500 dyn/cm²), uses a small d.c. motor running at constant torque to drive the cone. The range of the first instrument is 1 to 100 poises at 10 dyn/cm², and 10 to 100 poises at 100 dyn/cm², that of the second is 0.5 to 30 poises. Their accuracy varies according to the viscosity and anomaly of the fluid under examination, but the average is about ± 5%, which is quite adequate for many purposes. The examination of a sample with either instrument, including filling and cleaning, can be completed within two minutes.

A description is given of a simple method of eliminating the effect of stray capacitances between the test specimen and earth, for a certain class of Schering bridge measurements. The method involves only one simple switching operation and analysis shows that, using this method, the test specimen may remain connected to the circuit throughout, so that a usual source of difficulty, the variation in strays upon connexion, is avoided.

A temperature-limited diode in a bridge circuit is used to measure true r.m.s. voltage. The nominal voltage is at the zero position of a centre-zero meter and voltage variations are indicated by deflexions to the left or right. Long-term stability is to ± 0.2% of the nominal voltage and short-term variations of 0.1% may be detected. The effect of changes in ambient temperature is negligible. A typical instrument for the 230 V mains has a range of 215-245 V and consumes 10 W.

An instrument is described in the design of which special care was taken to avoid the incorporation of sources of principal errors. It consists of two independent systems, one a self-balancing weighing beam for recording rainfall, and the other a floatchamber for automatic emptying, this being suspended on the load-side of the weighing beam and acting as rainwater receiver. Extensive tests of a prototype model have shown it to be accurate to more than 1% of its range. It is self-cleaning during each emptying process.

A camera suitable for Weissenberg and other types of X-ray diffraction photographs is described. The special features are its simple and substantial construction, and the large diameter of filmholder and consequent large size of the photograph. The distance from focal spot to crystal is 100 mm and thus exposure times are particularly short. These advantages are gained by the sacrifice of the equi-inclination feature provided in the majority of Weissenberg cameras.