274 



Wadsworth — Determination of Specific 



A better arrangement for observing both the temperature T 

 and the temperature 6 would be thermo-elements placed respec- 

 tively in the heating chamber and in the calorimeter.* 



(c) New Method. — Returning now to the method of Rum- 

 ford, it is evident that we might indeed find such a value for 

 the external temperature that the maximum reading S n would 

 be the same as though there were no radiation. Thus in the 

 figure below (fig. la), if we let the ordinates represent tempera- 

 ture and the abscissas time, we have, when no radiation takes 

 place, the curve BC, whose equation is of the general form 

 x=f(log 6). Hence the maximum 6 is never reached. When 

 we consider radiation, however, we find that the curve BA 

 will, when /3 has a certain value between t and 6 (ft being 

 as before the temperature of the air), reach a true maximum 

 at A, which is equal to the theoretical maximum 6. At this 

 point the heat imparted to the water from the body just equals 

 the heat lost by radiation. 



The heat lost by radiation, dq r =zAS(6—fi)dx 



(9) 



The heat gained from the body, dq t =- S 1 (T —0)dx (10) 

 where <t is the coefficient of external thermal resistance ; S 



* A still more promising method of observing temperatures is the use of the 

 platinum thermometer recently brought to such a degree of perfection by the 

 work of Callendar. Either this, or the thermo-element (now immensely improved 

 by the work of Barus and others), ought on account of its small mass, rapid 

 response to change of temperature, and sensitiveness, to be far better adapted to 

 accurate calorimetric work than the mercurial thermometer now so commonly 

 employed. About the only advantage of the latter is its simplicity and cheap- 

 ness, — qualifications which determine its use in ordinary laboratory work, but 

 have far less importance in accurate research work. 



