HIGH TEMPERATURE MEASUREMENT 509 



image of the object whose temperature is to be measured is 

 formed by the telescope ; the comparison source is a standard 

 lamp, in which amylic acetate is burnt in a flame of stated 

 dimensions. In the newer forms of the latter instrument three 

 overlapping temperature scales are usually provided, the lowest 

 temperature on the first scale being 800 to 900" C. and the highest 

 measurable about 4,000'' C. In the Wanner instrument the 

 comparisons are made by adjusting to equal brightness the two 

 similar halves of a circular disc of light one part of which is 

 illuminated from the object sighted upon, and the other from 

 a small electric lamp attached to the instrument. The bright- 

 ness of the lamp, which is maintained incandescent from a port- 

 able 4-volt accumulator, is previously adjusted to the desired 

 amount by comparison with an amylic acetate standard flame. 

 The Wanner instrument differs from the other two in that it is 

 not an ordinary telescope, but a straight-vision spectrophoto- 

 meter and that no real image of the object on which it is sighted 

 is formed by the optical system. The chief advantages of these 

 instruments over the total radiation pyrometer are that they 

 can be used on smaller objects ; if required readings can be 

 taken through an interposed window, with very little alteration 

 in the results obtained. The Fery absorption type can be 

 employed for the determination of the temperature of the electric 

 arc or even of an incandescent lamp-filament. 



Another type of optical pyrometer much used for certain pur- 

 poses is that known in America as the Morse and in Germany 

 as the Holborn-Kurlbaum instrument. In the latter of these 

 a small incandescent lamp with a plain horseshoe filament is 

 placed at the focal-point of a short-focus telescope, sighted upon 

 the object whose temperature is to be measured. The current 

 through the lamp is supplied from a small portable accumulator 

 and is adjusted by a rheostat so that the bend of the filament 

 appears equally bright with the hot background, and at this 

 moment becomes indistinguishable from it. The magnitude of 

 the current is then read off" on a suitable deflection-ammeter, and 

 reference to a table gives the required temperature. In the 

 commercial forms of instrument a direct-reading temperature 

 scale may be provided in addition to the readings of current on 

 the ammeter, if desired. This type of instrument may be used 

 from about 600'' C. upwards and gives good results. The better 

 forms are provided with several of the celebrated incandescent 



