Modern Gasworks Practice

 242 MODERN GASWORKS PRACTICE pointer which moves across a dial. The instrument is calibrated so that the reading of the pointer on the dial indicates directly the temperature of the body on which the instrument is sighted. Focusing is effected in the same way fas with the Féry thermo-electric instrument already described. The three standard langes for this pyrometer are 500-1,100° C., 600-1,400° C., and 800-1,700° C. 1 GENERAL CONSIDERATIONS 2 })0 generally agreed that in point of convenience and SC0Pe of utility no types of pyrometer have attained so high a __) position as the resistance and thermo-couple instruments. The main consideration to be borne in mind is that the extent of accu-racy which is required. in the measurement of temperatures varies considerably with. the purpose in view. The gas engineer for his general works purposes does not need a pyrometer such as would be essential to the research chemist. He can well afford to sacri-fi.ce 2 or 3 per cent, of accuracy for more tangible considerations, FIFbry42'spiral SUCh as to withstand rough usage and bad conditions. In (4 times actual faet, it will not be considered uncomplimentary to say that the size)’ first care should be that of selecting an instrument which is “ fool- proof.” One of the most notable features in the progress of industrial pyrometry during the last decade has been the widespread introduction of base metal thermo-electric Pyrometers. This is mainly the result of the enormous increase in cost of platinum and other rare substances; but partly to the faet that base metal couples possess certain distinct advantages. One of these is the development of a large E.M.F., which enables more robust indicators and recorders to be used, and another is that the pyrometer itself may be strongly constructed and thus made less liable to injury in workshop use. Although they are limited as regards temperature range—the upper limit bping about 1,000° C. for continuous use—base metal pyrometers are suitable for a multitude of industrial purposes. The couples commonly employed are iron-cönstantan, two iron-nickel alloys of different composition, or two different nickel-chrome alloys. Attempts are now being made to devise a base metal couple for use at higher temperatures, the method which has been tried being to utilize the E.M.I1. developed by a junction in which one or both of the metals may be in the liquid state. This is rendered possible by the faet that tin, aluminium, copper, and certain other metals may be used up to temperatures approaching the boiling-point, which. ranges from 1,800° in the case of aluminium to 2,310° for copper, if the junction can be maintained. -A concise statement of the advantages to be derivod from the use of pyro-meters has been given by R. P. Brown,1who says that the extensive use ofpyrometers to measure or record high temperatures will serve (1) to eliminate guesswork as to 1 Proc. Faraday Society, November, 1917.