1892.] 



on Metals at Eigh Temperatures. 



505 



Fig. 1. 



ways, indicates the temperature of the coil, C. The coil itself may- 

 be adequately protected and exposed to temperatures which have 

 been determined by the air thermometer; the deflection of a suitable 

 (ditferential) galvanometer G, will then indicate 

 temperatures directly. For instance, the temperature 

 at which zinc boils has been accurately fixed at 

 940° C, and if the coil is heated in the vapour of 

 boiling zinc, the angle through which the galvanometer 

 mirror is deflected marks the temperature of 940° C. 



The Report of a British Association Committee 

 showed, in 1874, that the instrument is liable to 

 changes of zero, but Mr. H. L. Callendar has recently 

 (1887) restored confidence in the method which had 

 been shaken by the Committee. He has proved 

 that if sufficiently pure platinum wire be used, and 

 if the wire be carefully annealed and protected from 

 strain and contamination,* resistance pyrometers may 

 be made practically free from changes of zero even 

 when used at temperatures as high as 1000° C. He 

 attributes the changes of zero to which the Siemens 

 pyrometers are liable to the action on the wire 

 of the clay cylinder on which it is wound, and of 

 the iron tube in which it is inclosed. As the result of his experi- 

 ments he has introduced certain modifications, which render the 

 instrument not only trustworthy but very sensitive. He winds the 

 platinum wire on a thin plate of mica, and incloses it in a doubly 

 glazed tube of hard porcelain. He uses the zero method of measur- 

 ing the resistance ; but for these and other details of manipulation 

 his own very interesting papers must be consulted. I will only add 

 that I have had the pleasure of working with him in the Mint 

 Laboratory, and I am satisfied that at temperatures about 1000° 

 the comparative results afforded by his method are accurate to the 

 tenth of a degree, a result which would certainly have been deemed 

 impossible a year or two ago.f 



* 'Phil. Trans. Roy. Soc' vol. clxxviii. 1887, A, pp. 161-233, and vol. clxxxii. 

 1891, A, pp. 119-157; 'Phil. Mag.' vol. xxxii. July 1891, p. 104, and vol. xxxiii. 

 Feb. 1892, p. 220. 



t As this statement has been received with some surprise, it may be as well 

 to state briefly how this degree of accuracy and sensitiveness is attained. Tlie 

 resistance-box is compensated for changes of temperature, and chano-es of resist- 

 ance in the wires leading to the pyrometer are automatically eliminated. The 

 resistance itself is measured by a modification of the well-known Carey-Foster 

 method. The balancing resistance of the Wheatstone bridge employed, is com- 

 posed partly of resistance coils and partly of a bridge-wire along which a contact 

 key slides. The resistance of a centimetre of this wire is made to correspond to 

 the increase of resistance of the pyrometer produced by a rise of 1° C. The 

 galvanometer can easily be made sensitive to one-hundredth of a centimetre 

 of this bridge-wire, bo that one-tenth of a centimetre, which corresponds to one- 

 tenth of a degree, can, of course, be measured with certainty. 



Vol. XIII. (No. 86.) 2 m 



