510 SCIENCE PROGRESS 



lamps, some of which can be reserved as reference standards, 

 and used to check constancy of the relation between tempera- 

 ture and current in the working lamps. For measurement of 

 temperatures above 1,400° C. a system of mirrors forming a 

 weakening-device is placed in the path of the light in front of 

 the object-glass of the instrument, and by this means a second 

 scale extending from about 1,200° C. to 2,200° C. is obtained, with- 

 out risk of damage of the working lamps by over-running. This 

 type of instrument is perhaps a little easier to adjust than the 

 others, and can be used on a small object. It and the Wanner 

 however require accumulators, which are unnecessary with the 

 other patterns. None of these types of " optical " as distinct 

 from the " total radiation " pyrometer can be made to record nor 

 can their indications conveniently be transmitted to a distance. 

 Moreover they all involve a personal element in the setting, 

 which is not present with the " total radiation" pyrometer. 



Of the remaining available methods of measuring tempera- 

 ture only two need be mentioned, the calorimetric method of 

 Siemens and that depending on the use of fusible materials as 

 indicators, such as the well-known " cones " of Seger and the 

 Watkin " recorders." 



The calorimetric method involves the use of a cylinder of iron, 

 copper, nickel or other metal, which is heated in the furnace 

 whose temperature is to be measured, and then dropped quickly 

 into a calorimeter containing a definite amount of water. The 

 rise of temperature thus produced in the water is indicated 

 by a thermometer ; by a simple device involving a sliding- 

 scale, the temperature to which the specimen of metal was 

 heated is obtained without calculation. This method of measure- 

 ment, introduced by Sir William Siemens about forty years 

 ago, is still used successfully in many industrial processes. Its 

 practical upper working limit is however not much above 

 1,000° C. 



Seger cones and other similar heat-recorders are used 

 largely in pottery works, where the temperature distribution 

 over wide areas in a large furnace needs controlling at a number 

 of points. Usually three to six different samples of refractory 

 mixtures of definite melting-points are simultaneously exposed ; 

 from a subsequent examination of these after withdrawal 

 from the furnace the temperature is deduced. The method is 

 capable of giving results sufficiently accurate for many purposes, 



