Determining the Radiation Constant. 27 3 



draughts by loosely fixing under it a wide pad of cotton-wool, 

 held in position by a wire passing between the retort stands 

 supporting W, since otherwise the opening and shutting o£ 

 doors, &c, was sufficient to occasion an irregular and jerky 

 motion of the galvanometer-coil. 



During the boiling o£ the water, the silver disk was 

 covered by a screen with one edge resting on two thin corks 

 to prevent actual contact between the disk and the screen. 

 This screen was made by taking a piece of sheet nickel, 

 approximately twice the size of the board W, doubling it 

 into two with the dull side inwards and, after placing a layer 

 of cotton-wool between the folds, wiring the two folds loosely 

 together. 



When steam had passed for some time through the 

 vessel A, the latter was placed on the screen. At a particular 

 instant the screen was removed, A was lowered over the disk 

 and some six to ten readings of the position of the cross-wire 

 image were taken at intervals of five seconds (in the majority 

 of cases). 



In order to find the temperature equivalent of the 

 galvanometer deflexions, the silver disk was kept at a constant 

 temperature, and the deflexions of the galvanometer were 

 noted when the temperature of the other junction was altered 

 by a known amount — subsequent reference is made to this 

 point. 



Before beginning an experiment it is necessary to ascertain 

 that the two thermo- junctions are at the same temperature 

 or to know the difference of temperature between them. In 

 these experiments the following procedure was adopted : — 

 By insertion of the shunt-key S the galvanometer was 

 short-circuited and its zero-position found. Did. the de- 

 flexions, consequent to the removal of the key, indicate that 

 the temperature of the disk was too low, the hand was held 

 over the disk until the cross-wire image returned to its 

 initial position of rest ; whilst if the temperature were too 

 high, ether was poured on to a piece of cotton-wool resting 

 on a crucible lid, and this was allowed to evaporate in the 

 neighbourhood of the disk. 



The results of a number of experiments extending over 

 varying ranges of temperature are given in Table I. In the 

 last column but one are given the values of the initial rate 

 of change of temperature, whilst the last column contains the 

 values found for a . 10 5 . It will be noticed that in some of 

 these experiments alcohol and acetone were passed through 

 the radiating vessel and afterwards condensed in a condensing 

 worm (not shown in fig.). But subsequent experiments 



Phil. Mag. S. 6. Vol. 10. No. 56. Aug. 1905. T 



