Porcelain with Rise of Temperature. 



93 



For the determination of the fixed points R and R 100 , the 

 apparatus was more cumbersome than that used for smaller 

 pyrometers. In determining R l0 o> steam from a boiler was 

 led into a metal tube about 6 cm. in diameter surrounding 

 the porcelain tube. The metal tube was surrounded by a 

 similar wider tube 10 cm. in diameter and co-axial with it. 

 The steam, passing along the inner metal tube, returned 

 through the space between the two tubes, forming a jacket, 

 and thence pnssed out to a condenser. A siphon-gauge 

 showed any excess of steam-pressure within the apparatus 

 above the atmospheric pressure. The jacketed steam vessel 

 and the tubes leading from the boiler were carefully packed 

 round with cotton-wool and hair felt. The steam-vessel was 

 slightly inclined to the horizontal, so that the condensed water 

 might drain away. It was found that with this arrangement 

 the readings very soon became steady when steam was passed 

 into the apparatus ; and the values of R 100 deduced from 

 experiments on different days and under different barometric 

 pressures agreed satisfactorily with one another. 



The ice-point E was determined by immersing the porce- 

 lain tube in a mixture of pounded ice and water contained in 

 a vessel formed from the larger of the metal tubes used for 

 the steam-point determinations. 



The sample of platinum wire, from which the piece used in 

 these experiments was cut, is known to have a value of S, in 

 Calendar's formula, from 1*50 to 1*51. The value $=1-505 

 was assumed, and thus a direct determination of the resistance 

 at the temperature of boiling sulphur was avoided. An error 

 of *01 in 8 causes an error of less than 1° in the calculated 

 value of t at 1000°. 



After a careful determination of the ice and steam points, 

 the tube was heated as follows : — 



Table I. 



Number 

 of Expt. 



Date. 



Maximum 

 Temperature. 



Number 

 of Expt. 



Date. 



Maximum 

 Temperature. 



1 



March 13. 

 „ 17. 

 „ 21. 

 » 23. 

 ,, 24. 



77° C. 

 319 

 650 

 640 

 740 



6 



March 29. 



„ 30. 



April 3. 



„ 7. 

 8. 



810° C. 



824 



828 



821 



840 



2 



7 



3 



8 



4 



9 



5 



10 







