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 + ct2
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.
Fig. 60.—Siemens’ Optical Pyrometer, on Stand.
