Table of contents

The interferometer is of the Rayleigh two-slit type. A glass plate partly covered by the film to be measured is placed so as to cover one slit and the resulting fringe is a measure of the phase retardation due to the film. The shift is determined photoelectrically using a homodyne detection system to locate the fringe minimum. A sensitivity of λ/7000 is obtained.

The fluxmeter described uses a taut-suspension galvanometer, the torque of which is compensated by positive feedback. This latter is furnished by a servo-operated potentiometer, the wiper of which follows the motion of the galvanometer light spot. The device is an adaptation of a commercially available recorder and permits recording of flux-variations as low as 100 Mexwell-turns per second.

The design and the method of construction of electric-current shims for the Birmingham cyclotron are described. Measurements are given showing the effects of these on magnetic-field distribution. Representative beam-current measurements are given which show that with shim-currents up to 500 A, the method was successful in making possible the acceleration of H⁺ or ³He⁺⁺ ions with about the same efficiency as the D⁺ ions for which the permanent steel shims were adjusted.

The paper describes a scintillation counter apparatus used for single-crystal X-ray diffractometry. The variation of the signal/noise ratio with photomultiplier operating potential has been investigated for the Du Mont 6291 tube, and it is shown that the resolution of the signal pulses from the noise background improves as this potential is decreased. Signal resolutions of 26, 29 and 43% have been obtained with AgKα, MoKα and CuKα radiations respectively, the mean number of photoelectrons released per keV incident quantum energy being 6. Stability criteria which will help to ensure the overall uniformity in the counter response are suggested.

A circuit for controlling the r.m.s. value of an a.c. signal is analysed and practical results are discussed. The speed of response is such that for rates of change of 2.5% per second the output is controlled continuously within 1%. An alternative arrangement for maintaining, at a preset level, the peak value of repetitive pulses is also given.

The instrument described is a plastic-phosphor scientillation-type detector which can be used to monitor the radio-activity of liquid and gaseous effluents. With liquid effluents, it can detect activities of about 1.5 × 10^-6 μc/cm³ of 0.7 MeV γ-radiation, and for gaseous effluents, 1/280 m.p.c. of ⁴¹A in air. The experimental determination of the optimum sensitivity of the detector system is described and results are given. The system provides a continous day-and-night record of the activities monitored. Also, it gives automatic warning of dangerously high levels of activity and of a failure in its own electrical system.

A method of tracing metal pipes, involving the amplification of the voltage induced in a special type of search coil by transient currents injected into the pipe, is described. The search coil consists of two identical flat coils mounted side by side in the same plane, and so connected that the output voltage is equal to the difference of the voltages induced in the two coils. An audible signal is obtained in headphones after amplification of the output voltage. It is found that, with this twin coil system, the signal strength varies approximately as 1/R², where R is the perpendicular distance from the pipe to the search coil, and that a more reliable performance is obtained compared with the usual single type of search coil.

The instrument described, though not being the answer to all vibration-measuring problems, has characteristics which make it ideal for the measurement of vibration amplitudes at frequencies below 50 c/s and for the calibration of other transducers. Also, because its frequency range extends down to zero, the instrument can be used to detect and measure small displacements.

Constructional and performance details are given of a photoelectric spectrophotometer erected for the measurement of the optical transmittances of small solid specimens, in the range above 900 Å. The instrument incorporates a one-metre radius, 600 lines/mm concave grating, and scanning of the spectrum across the exit slit is performed by a linear movement of the centre of the Rowland Circle. The mechanical and electrical arrangements permit rapid measurements, quick exchange of specimen and the direct reading of transmittance on a 0-100 scale.