738 TRUEBLOOD. 



The most ob\'ious defect in the type of plug shown in Fig. 1 is, of 

 course, the opportunity for heat leakage afforded by the wide sepa- 

 ration of the two thermometer bulbs. Nearly half of all the throttling 

 has taken place before the steam enters the cross-channel, in which 

 the temperature of the steam is consequently considerably less than 

 that of the oil. This defect was not unnoticed in designing the appara- 

 tus, but its importance was much underrated. It was expected that 

 the fairly good thermal insulation of the steam while in the cross- 

 channel would prevent heat leakage of inconvenient magnitude (the 

 soapstone blocks of Fig. 1 are about two inches thick and over four 

 inches wide), and the first practicable apparatus used was built in 

 this form, chiefly because it could be easily handled and because the 

 design permits an easy and rapid interchange of the (resistance) 

 thermometers for the purpose of eliminating their normal difference 

 of resistance. Soapstone, however, is not a particularly good heat 

 insulator, and it was found impossible to obtain reliable values of the 

 Joule-Thomson coefficient with plugs of this type without an excessive 

 amount of variation in the conditions of flow and pressure drop at each 

 pressure and temperature of measurement. 



2. Radial Flow Apparatus. 



a. General remarks on the radial fioio plug. 



The theory of the radial flow porous plug has been discussed in a 

 paper by Burnett and Roebuck.^ One of these authors has since 

 applied it to the experimental determination of the mechanical equiva- 

 lent of heat.^ A radial flow plug was also used by Regnault in some 

 porous plug experiments on air. He regarded his results as very 

 unsatisfactory and the time spent in securing them as wasted. So 

 far as is known to the writer, no other experimental results obtained 

 with the radial flow plug have been published. Briefly, the radial 

 flow plug consists of a thin cup of porous material of low thermal 

 conductivity — porcelain, for example — and of a length some 6 or 8 

 times its diameter (see Fig. 2). The fluid under experiment is made 

 to flow through the cup from the outside to the inside. The low-side 

 thermometer bulb is located near the bottom of the cup on the inside. 



2 E. S. Burnett and J. R. Roebuck, Phys. Rev. (1) 30, 529 (1910). 



3 J. R. Roebuck, Phys. Rev. (2), 2, 79 (1913). 



