Passing Isothermally from Solid to Liquid. 7 



(as before) when the thermo-couple shows no difference of the 

 temperature between the plates. 



8. External conductivity. — To determine the radiation con- 

 stant h 1 I used two methods, in one of which the cold plate 

 (jD) was raised in an environment hot above and cold below; 

 and in the second of which the hot plate (D) was raised in a 

 uniformly cold environment. The latter is essentially that of 

 Weber. 



The environment being at any constant temperature, I 

 removed the spacers t, t, and placed the copper plates in con- 

 tact. A strong current of water circulating in FF, kept the 

 plates at the same temperature differing from that of the envi- 

 ronment. By aid of threads fastened to ss, and a wire, passing 

 through JV, the plate D was now raised and kept suspended by 

 a clamp on the outside. During all this time the differential 

 thermo-couple was in place and changes of temperature of the 

 plate were thus registered. 



Relatively to the slow external conduction, the suspended 



plate DD is always an isothermal region. Hence if M 1 be 



the mass, c l the specific heat, and u the temperature excess of 



the plate at the time t; if F x (top and sides) be the surface 



toward the hot environment and F (bottom) the surf ace toward 



the cold environment ; if finally z be the temperature excess 



of the hot environment and h 1 the external conductivity, then 



M&du = h^F^z-u^dt - h^Fudt, or du/dt + h i (F 1 .+F)u/M 1 c 1 



= h^F^z/M^^ a differential equation which after integration 



leads to 



. FrlO-u hO. 



In ' ' lA = i{t—t') .... 1) 



F x zjO—u M x c x x ' K ' 



where F-\-F=0, and where u and u' correspond respectively 

 to t and t' . 



In this way I obtained the results of Table 1, where z' is the 

 initial temperature of the cold plates, and hence z+z' the 

 actual temperature of the environment, and u+z' the actual 

 temperatures of the suspended plate at the consecutive times. 

 The table further contains dt=t — t' and o log(F 1 z/0— u) = 

 'IM \n{F 1 z/0-u')/{F,z/0-u\ and finally the values of h x . 











Table 1. 









External conductivity. 



Complex 



Cold plate 



raised. 





10 5 x 





Mean 



environment. 





Mean. 



St 



(5 log {Ft/ O- 



■u) 



u + t' 



t' t + t' 



hi x 10 1 



hi x 10 7 



sec 







°C. 



°G. °C. 







180 



1811 





6-6 



5-7 30-5 



1072 



1082 



180 



1766 





7-6 



5-7 30-5 



1045 





120 



1270 





8-5 



5-7 30-5 



1130 





180 



1878 





7-3 



5-7 29-9 



1112 



1040 



180 



1792 





7-8 



5"7 29-9 



1061 





180 



1685 





8-7 



5-7 29-9 



997 





180 



1661 





9-5 



5-7 29-9 



989 





