On the Heat dissipated by Platinum at High Temperatures. 403 



readings over any convenient range. The other two constants depend 

 on the length of the stoppage at the lowest point of the range, and the 

 relation between the rates of the lowering and the recovery of pressure. 

 The results calculated in this way show a very satisfactory agreement 

 with the Kew verifications. 



The investigation being principally intended to increase the useful- 

 ness of the existing Kew test, and to show where it is most in need 

 of amplification or amendment, attention is primarily given to the 

 defects of aneroids. It is hoped that the increased knowledge of these 

 defects will enable rules to be framed for the rejection of aneroids, and 

 that in this way it will be made worth while for makers to improve 

 the instrument. , The large differences brought to light between 

 different aneroids, show that the means of markedly raising the average 

 are already at the makers' disposal if they choose to utilise their 

 knowledge. The present enquiry also shows clearly how the effects 

 of tentative improvements may best be ascertained. The method of 

 utilising aneroids to the best advantage in determining mountain 

 heights is not formally considered, but a variety of the results should 

 nevertheless be found of immediate service by any traveller of intel- 

 ligence who has this object in view. 



" On the Heat dissipated by a Platinum Surface at High Tem- 

 peratures." By J. E. PETAVEL, 1851 Exhibition Scholar. 

 Communicated by LORD RAYLEIGH, F.R.S. Received 

 May 19, Read June 9, 1898. 



(Abstract.) 



The first part of the paper refers to the emissivity of a bright 

 platinum surface in air and in other gases. 



The temperature measurements are based on the researches of 

 Callendar and Griffiths, confirmed by the recent determinations of 

 Heycock and Neville. To check the calibration of the thermometers 

 at higher temperatures, the melting point of palladium was used. A 

 number of the curves are extended to 1779 C. by a direct measure- 

 ment of the emissivity of platinum and palladium at their melting 

 points. 



The platinum wire, which served at the same time as radiator and 

 thermometer, was Oil 2 cm. in diameter. It was placed in the axis 

 of a vertical glass cylinder, which formed the enclosure. 



The effects produced by a change in the size, shape, material, and 

 temperature of the enclosure and in the position and diameter of the 

 wire are also studied. 



The temperature is expressed in degrees centigrade, and the emis- 

 sivity in C.G.S. units. 



