THE MEASUREMENT OF HIGH TEMPERATURES 237
RADIATION PYROMETERS
The Féry radiation pyrometer is made in three patterns; for portable use, for continuously indicating, and for graphically recording temperatures. Each pattem has a telescope which. is focused on the hot Ibody, the heat rays being received on a concave mirror and brought to a focus on a small thermo-couple. The electro-motive force produced by the consequent heating at one junction of the thermo-couple is measured on an indicating or recording galvanometer calibrated to give
direct readings in temperature.
Fig. 137 illustrates the portable pattern. The outfit consists of a telescope, indicator, tripod, leads, and travelling case. The indicator, which is fitted with a double-pivoted moving coil, has a high sensitivity, but is at the same time of robust construction.
The indicator may be calibrated to cover any particuJar range of temperature desired from 500° C., but the standard ranges which will be found to meet most requirements are as follows :—
500-1,100° C. 600-1,400° C. 800-1,700° C.
The optical arrangement of the telescope is illustrated. in section in Kg. 138. The heat rays A from the furnace are received on the concave mirror C and are brought to a focus at N. Looking through the eye-piece E, the observer sees an image of the heated body in the small mirror M, and is able to direct the telescope on the exact spot of which the temperature .is required. The small sensitive thermo-couple is situated just behind a small hole in the mirror M, and
Fig. 137.
becomes heated by the rays passing through this hole.
An ingenious device is fitted on all telescopes to make the operation of focusing the instruments as simple as possible. The mirror M, in which the observer sees tlie image of the heated body, consists of two small semicircular wedge-shaped mirrors fixed together. If the instrument is correctly focused, the appearan.ee is as shown in the centre diagram of Fig. 139, in which. the outer circle represents tlie mirror, the shaded portion the reflected image of the hot body, wliile the black centre is the sensitive element of the pyrometer. If the focus is too short, the image appears split in two parts, as in the left-hand diagram, whilst if it is too long, the appearance