790 TRUEBLOOD. 



error is considerably smaller than this. It is true that it is necessary 

 to suppose the percentage extrapolation error with plug S to be only | 

 to Ys of this error with plugs Ul or U2, in order thereby to account for 

 an error of ^ per cent, in what we have called the true /jl. But it is 

 extremely' doubtful that the plots for plugs Ul and U2 can be regarded 

 as straight within experimental error, and therefore difficult to say 

 by how much the extrapolation error wuth these plugs might reasonably 

 be expected to exceed that with plug S, for which the rectilinearity 

 of the plot is more secure than for any other plug. Moreover, the 

 large values of the percentage extrapolation error for plugs Ul and 

 U2 are due to the small slopes of the plots and would be materially 

 changed by comparatively small changes in these slopes. 



5°. Nothing whatever as to the legitimacy of the assumption that 

 the A of equation (2a) is zero can be inferred either from the straight- 

 ness of a given plot or the smallness of its slope, and this assumption 

 has exactly the same validity in the case of plug S as it has in the case 

 of any other plug for which the value of the exponent r of equation (4) 

 is the same. But certainly it is not easy to see how or why a heat 

 leak per unit time varying as the 1.75 power of the pressure drop — 

 practically as the 1.75 power of the temperature drop — and of any 

 appreciable magnitude, could be present. 



6°. The effect of failure to correct for the pressure coefficient of /i 

 probably is to make the extrapolated fx too small, but not, it is believed^ 

 by an amount as great as 0.3 per cent. (See Section IV, 2, h.) 



7°. The precision of the temperature measurements is at least as 

 good as 5 per cent., and that of the pressure measurements ^ per cent.^ 

 or even better at the higher flows. 



/. MeasuremenU of fxCp and the calculation of yu frojn them. 



To measure directly the value of the product fxCp, it is necessary to- 

 supply energy to the fluid during its passage through the plug, of an 

 amount sufficient to prevent the ordinary Joule-Thomson drop of 

 temperature. In the case of such axial flow plugs as have been used 

 in this work, this energy is supplied electrically by means of the 

 heating coil shown in Fig. 1. If the supply of energy is just sufficient 

 to maintain the low-side temperature equal to the high side tempera- 

 ture, it may be assumed, in the absence of such effects as varying 

 kinetic energy in different parts of the plug, that the temperature of 

 the steam is that of the oil-bath during the whole of its passage through 



