A New Form of Mixing- Calorimeter. 247 



Professor Ayrton and I have used these strips in weighing- 

 machines, instruments in which forces require to be measured, 

 and instruments in which small motions require to be mag- 

 nified. We are now using the double-twisted strip of constant 

 length, but with initial pull in it, as a thermometer and as a 

 galvanometer. It is my opinion that it only requires to be 

 better known to be largely used ; and although I seem to 

 have spent rather in vain a considerable amount of time in 

 trying to get a working theory, I do not think the time has 

 been really wasted. It is no mere curious puzzle, it is a 

 problem of practical importance. 



The two great difficulties are — (1) What is the law of p over 

 the section? (2) to what extent is it wrong to regard the 

 section as resisting untwisting with the moment 



o 



that is, to what extent has the original twisting given initial 

 shear strains to the material? 



XXVI. A New Form of Mixing-Calorimeter, 

 By Spencer U. Pickering, M.A.* 



[Plate VII] 



THE methods which have been employed in recent calori- 

 metric work for mixing two liquids are two in number. 

 Berthelot (Mec. Chim. i. p. 171) places one liquid in the 

 calorimeter, and adds the other from a flask, which is handled 

 by the help of wooden clips; while Thomsen (Thermochem. 

 Untersuch. i. p. 19) has a metal calorimeter for each liquid, 

 one of them being placed above the level of the other, and 

 communicating with the lower one by means of a metal tube, 

 which is stopped by a plug till the temperatures have been 

 ascertained. Berthelot's method is inapplicable in cases where 

 delicate thermometers with large bulbs have to be employed ; 

 and there are various other objections both to his and to Thorn- 

 sen's method, of which the most serious is that the two liquids 

 before being mixed are not at the same temperature, and the 

 results obtained are, therefore, dependent, not only on the 

 accuracy with which the two thermometers have been com- 

 pared with each other, but also on an accurate knowledge of 

 the heat-capacities of the two liquids. 



In devising a new form of mixing-calorimeter, the chief 

 improvement which I aimed at was to obviate this source of 



* Communicated by the Author. 



