Table of contents

A heat-flow meter is described having the form of a small disk of tellurium-silver alloy with copper-gauze coating on its two sides. This is placed with its plane at right angles to the flow of heat to be measured, creating a small proportionate difference of temperature between the two sides. The output of the disk is 2.0 μV. for a flow of 1 B.TH.U./ft.²/hr. The internal resistance including leads is 1 ω. The disk is connected by fine wires through a terminal block some distance away to a galvanometer which registers the thermo-e.m.f. due to the difference in temperature of the two tellurium-copper junctions. The disk is calibrated by placing it in a measured heat-flow through a number of layers of plush. Experiments carried out in a room-size steel-walled chamber with one or more of its walls heated gave an approximately correct figure for the heat imparted to the injected air current, the volume and change of temperature of which were measured.

An infra-red image-converter tube is associated with a form of disappearing-filament optical pyrometer to enable readings to be taken at temperatures as low as 350° C. By use of a strip of paper illuminated by a lamp, instead of the normal pyrometer lamp filament, a reproducibility of 1-2° C. has been secured in the temperature range from 350 to 700° C. The observer's eyes must be dark-adapted in varying degree according to the temperature to be measured. At the lowest temperature about 15-20 min. must elapse after the observer enters the darkened room, from daylight, before he can make accurate readings. A rough calculation of the effective wave-length of the pyrometer gives a value of 1 μ and the effect of variations in this value is discussed.

Corrections to 1949 J. Sci. Instrum.26 321.

A scale-of-ten unit, employing two pentode and five diode valves, has been developed along the lines of the Miller integrating circuit. It is primarily designed for connexion to the output of a commercial scale-of-100 unit to provide a scale-of-1000 with meter indication of the number of hundreds. A number of such units, suitably modified, could be used in cascade to provide any desired scaling factor. Counting is achieved by the collection of discrete quantities of charge on a capacitor. A lower limit to the permissible counting rate, of about one count in 20 sec. at the output, is imposed by leakage at the capacitor. The mechanical counter sets an upper limit of 9 counts/sec. at the output; at this rate counting losses in the circuit are negligible.

During the study of the effect of freezing on certain building materials, it became necessary to develop an apparatus to measure small linear changes in two dimensions. In the apparatus described, movement of the specimen is magnified by the change in the angle of reflexion of a beam of light incident on a system of rotating mirrors. A device is incorporated to translate horizontal movements of the specimen into vertical movements on the extensometer.

The history of previous attempts to generate artificial dust clouds is reviewed and the technical difficulties are analysed. A new method is described, in which the dust is packed into a cake, of which the surface is then scraped off in a very thin layer into an air stream. Details are given of the construction and performance of a new mechanism based on this principle. Concentrations can be indefinitely kept constant within a range of ±20% and the mechanism is suitable for a large variety of fine dusts.

A hollow cylinder of manganese is used as a detector of thermal neutrons which are generated by the moderation in a surrounding water tank of fast neutrons from a source placed within the detector. The activation of the detector is investigated as a function of the position within it of a very small neutron source and found to be insensitive to this position. The detector then provides an accurate and sensitive means for the comparison of neutron sources of very different sizes, such as a large radium plus beryllium mixed source and a small radium-beryllium-fluoride neutron standard source.

A small helium liquefier and portable cryostat are described. This apparatus enables a laboratory producing liquid helium in amounts of the order of 50 c.c. or less to transport the liquid to any place where it may be required. Hitherto small liquefiers have had cryostats inseparable from the main liquefying systems. An account is given of the use of a portable cryostat for the demonstration of low-temperature and liquid-helium phenomena to a large audience.

Information about the stability of inductance standards that has been acquired at the National Physical Laboratory during the last thirty years or so is reviewed, and it is shown that the present working standard of mutual inductance, which is of the oil-immersed type and was constructed in 1937, has remained constant within ±2 parts in 100,000 throughout the twelve years it has been in use. The stability of the primary standard is considerably better, but some standards of the ordinary construction have changed by a few parts in 10,000 in a few years. Data are also given for standards of self-inductance.

An arrangement for spectroscopy in the 3 μ region (near infra-red) is described which is particularly suited for measurement of small specimens of oriented materials with polarized radiation. The spectrometer is fitted with a lead-selenide cell, using radiation interrupted at 800 c/s. A magnified image of the specimen is projected on the spectrometer slit by a mirror arrangement, enabling specimens measuring 0.2 by 0.5 mm. to be used.

Several special techniques are described, including the use of evaporated selenium to parabolize a spherical mirror.