=Cambridge Optical Pyrometer.=—During the recent war the manufacture of pyrometers of this type was taken up by the Cambridge and Paul Instrument Company. The external form of the Cambridge optical pyrometer is shown in fig. 58, in which an observer is shown using the instrument, the accessories consisting of a 4-volt accumulator, an ammeter, and an adjustable resistance for regulating the current through the electric lamp used for comparison; and a standard amyl-acetate lamp for adjusting the electric lamp to the correct brightness. The scale is marked on a circular disc, and direct readings are obtained from the position of a pointer which rotates with the analyser. By interposing a monochromatic glass to dim the source, the range of the pyrometer can be modified; and instruments are provided in four ranges: 700°-1400° C.; 900°-2000° C.; 1200°-2500° C., and 1400°-4000° C. The Cambridge optical pyrometer has proved a useful instrument in skilled hands, and has been found of great service in the steel, glass, and pottery industries. Trained observers have found it possible to detect a difference of 10° C. at the region of 1900° C. The adjustment of the two fields to equality, however, involves a judgment which varies with different observers, and in practice it is advisable for one individual to be entrusted to take all readings. [Illustration: FIG. 58.—CAMBRIDGE OPTICAL PYROMETER.] =Holborn-Kurlbaum Pyrometer.=—In the optical pyrometers previously described a constant standard is used, and the brightness of the light from the source varied until equality is obtained. The idea of varying the brightness of the filament of an electric lamp until its colour matched that of the source, and deducing the temperature from the current taken by the lamp, was due to Morse, who used a filament in the form of a flat spiral, heated by a battery of E.M.F. 40 volts. This spiral was placed in a metal tube and interposed between the eye and the heated object. The Holborn-Kurlbaum pyrometer, as made by Siemens, is a refinement of that of Morse, and capable of reading over a more extended range. In fig. 59, L is a small electric lamp with a hairpin filament, as shown at A. This lamp is placed in a telescope, so that the filament is in the focus of the eye-piece and is lighted by a 4-volt accumulator, in series with which is a rheostat, R, and a milliammeter, M. The heated source is focused by moving the object-glass of the telescope, and both lamp and source are viewed through red glass placed in front of the eye-piece, D. The rheostat, R, is then adjusted until the tip of the filament is indistinguishable from the background, which is illuminated by the source. If the lamp be too bright, the filament will appear as a bright line; if duller than the source, as a dark line; and when equal to the source it will merge into the background. When equality is obtained, the milliammeter is read, and the temperature deduced from the current taken by the lamp.