238
MODERN GASWORKS PRACTICE
is similar but reversed, as in the right-hand diagram. Turning the knurled head attached to the pinion F causes the upper and lower half-images to slide on one another, so that it is comparatively easy to arrive at the correct focus.
The size of the body focused on, and the distance of the telescope from it,
do not, within wide limits, affect the readings. It is, in faet, only necessary for the body sighted on to be of such a size that its image will overlap the sensitive element on all sides.
With the Féry radiation Pyrometer the size of the object sighted on must be at least 1 inch in diameter for every 2 feet of distance be-tween the telescope and the object.
The thermo-c o u p 1 e s supplied with these instruments are of three different
Fig. 138.—The Féby Radiation Thermo-couple Pyrometer, types, namely :—
(a) Silver and constantan, for temperatures up to 800° C.
(&) Platinum and platinum-
iridium, for temperatures up to 1,000° C.
(c) Platinum and platinum-rhodium, for temperatures up to 1,400° C.
Fto. 139.
THE FOSTER RADIATION PYROMETER
Of the newer radiation pyrometers the Foster instrument (Fig. 140) presents some novel features. In tlie first instance, it is of the fixed focus type and, therefore, requires no adjustment. The receiving tube is pointed at the hot body and receives a portion of the radiant heat therefrom. The radiated heat received is focused, by means of a special mirror, on the junction of a minute thermo-couple. The E.M.F. generated at the thermo-couple is carried by leads to a separate recorder. The operation of the pyrometer is independent of the distance from the object, within. certain. fairly wide limits. However, a sighting device is incorporated in the tube,