[Illustration: FIG. 59.—HOLBORN-KURLBAUM PYROMETER. SECTION.] The relation between current and the temperature of the filament varies with each lamp, but is in all cases represented by a formula of the type C = _a_ + _bt_ + _ct^2_ where C = current, _t_ = temperature in degrees C., and _a_, _b_, and _c_ are constants depending upon the lamp used, and which can be determined by making a number of observations at known temperatures. The instrument is calibrated in this manner by the makers, and a scale affixed from which temperatures may be read corresponding to observed currents. When the temperature of the source exceeds that of the standard at maximum current, an absorbing device, E, consisting of two prisms of darkened glass, with their reflecting faces parallel, is placed over the end of the telescope, so as to reduce the intensity of the source below that of the lamp. A separate calibration is performed with the absorber in position, and a second temperature scale provided, from which readings are taken when the absorbing device is used. Fig. 60 represents the instrument as made by Messrs Siemens, for use in a fixed position, the telescope, milliammeter, and rheostat being mounted on an upright supported by a tripod, and the current obtained from a portable accumulator. A second form (fig. 61) is designed for use in cases when observations at a number of different places are required, the rheostat being mounted on the telescope, and the milliammeter contained in a leather case provided with shoulder-straps. [Illustration: FIG. 60.—SIEMENS’ OPTICAL PYROMETER, ON STAND.] [Illustration: FIG. 61.—SIEMENS’ OPTICAL PYROMETER, PORTABLE FORM.] The adjustment in this pyrometer is simple, and the condition of equality sharply defined. Whereas, in matching the colours of two contiguous fields, separate observers may disagree to an extent representing 40° C. or more, a divergence of 10° C. is seldom exceeded when different operators adjust the tip of the filament to extinction. In a special test to decide this point, the author compared the observations of five persons, some trained and others untrained, with the result that all agreed to within 10° at a steady temperature in the vicinity of 1200° C.; and in this respect the Holborn-Kurlbaum pyrometer is superior to other forms of optical pyrometer. The continuous accuracy of the readings depends upon the permanence of the standard lamp, which is ensured by over-burning for 20 hours, after which the lamp may be used at its proper voltage for a long period without further change. As used for occasional readings in the workshop, such a lamp will last for a year or more without varying in brightness by an amount representing 10° C. at a temperature of 1800° C. When a new lamp is used, a fresh calibration is necessary; the makers, however, in such case send out a new temperature scale with the lamp.