The theory of the katharometer

[ Historical Note by G. A. Shakespear, M. A., D. Sc. —In September, 1915, at the request of a member of the Board of Invention and Research of the Admiralty, I undertook to devise an instrument capable of giving automatic indication of the presence of hydrogen in small quantities ( e. g ., 1 or 2 per cent.) in air. The well-known surface-action of palladium and platinum wires suggested itself as a phenomenon obviously adapted to the purpose. The wire was used as two arms of a Wheatstone bridge, one of these arms being protected from the gas by a thin glass tube, the other being exposed. When a sufficiently great current of electricity was passed through the bridge, the exposed arm rapidly increased in temperature owing to surface combustion. The temperature, however, was liable to rise dangerously high if the hydrogen were present in suitable quantity, and, as safety from explosion was indispensable, this method was abandoned. The same apparatus was then applied with a much lower current, and with the wires consequently at a much lower temperature, to make use of the increase in thermal conductivity of the gas due to the admixture of hydrogen. This arrangement was found to be unexpectedly sensitive, and the method was adopted for the desired purpose. As the instrument was primarily intended to measure the purity of the air, the name “katharometer” was given to it. In its final form, the katharometer consisted of two small helices of thin platinum wire (about 0·001 inch diameter), enclosed each in one of two cells in a copper block. The arrangement will be readily understood from a reference to the accompanying figure (fig. 1). Each helix was mounted in a small frame, consisting of a loop of copper wire soldered to a ring of copper. This ring was fitted with an insulating plug, through which the lead wire, also of copper, was introduced. One extremity of the helix was soldered to the lead and the other to the distal end of the loop. The outer part of the lead passed through a plug of rubber fitting into the cell, and over this rubber an ebonite plug was pressed down by a screw collar or nipple. Thus the rubber filled tightly the upper part of the cell, and access of air or gas could only take place by diffusion through the rubber. This diffusion is a slow process, and, for the purpose for which the katharometer was originally intended, such a joint was sufficiently nearly gas-tight. Minor improvements in detail were afterwards introduced. Both cells were similarly fitted, but whereas one was thus hermetically sealed, the other communicated with the outer atmosphere through three small holes. The resistance of each helix was about 8 ohms when cold, and the main working current in the bridge was usually 0·100 ampère; this was sufficient for most purposes, and gave the wires a temperature about 15°C. above that of the block. The remaining arms of the bridge were of manganin wire.