Table of contents

A slide-wire potentiometer, modified for the purpose of reducing the disturbing thermoelectric forces, is described. The potentiometer has two parallel manganin wires. A manganin roller connecting one point of the resistance wire with an adjoining point of the parallel wire constitutes the potential terminal at the wire. Thus sliding contacts and the use of different metals at the contacts are avoided. The junctions between the manganin wires in the potentiometer and external copper wires are connected to a number of copper discs with good thermal contact although they are electrically insulated. This arrangement considerably reduces the temperature differences between the various points of connexion. A simplified potentiometer for measuring small resistances only is described. Finally a design of a switch free from thermoelectric forces is given.

Some improvements in the integrating photometer are described; mainly the substitution of a mechanical clutch with a magnetic release for the original magnetic clutch.

The paramagnetism of oxygen (commonly present as an impurity in liquid nitrogen) may lead to rises of temperature, due to various causes, when the liquid is placed in a magnetic field. The dependence of these upon the degree of purity of the liquid nitrogen and the magnetic field strength is considered. A simple and sensitive method of measuring the oxygen content of a sample of liquid nitrogen is described.

The construction adjustment and calibration of a polarization spectrometer provided with a Sénarmont compensator and an ellipticity half-shade system is described, followed by a brief survey of methods of preparation and examination of surfaces when determining optical constants or surface films.

A drum camera is described which is used in conjunction with a cathode ray tube for recording the wave form of "atmospherics". The patterns are registered as departures from a very closely pitched helical zero line, and records equivalent to those obtained previously with 1000 ft. of 35 mm. cine film can now be made on 1 m. of paper 10 cm. wide.

Corrections to 1937 J. Sci. Instrum.14 37.