90 



Mr. Henry Crew on a New Method 



for one to make his observations. The writer offers the 

 following solution : — 



Take the body within which it is desired to maintain a con- 

 stant temperature, and wrap it very closely with a fine 

 covered w T ire having a high specific resistance. The wire 

 should not be of iron, or any substance which is liable to 

 suffer large permanent changes of resistance by oxidation or 

 heat. The body having been wrapped as completely, as 

 closely, and as uniformly as possible, a constant electric cur- 

 rent is now passed through it. By this means can be prac- 

 tically developed the same amount of heat per second over 

 every unit of surface of the body. 



If now the body is surrounded by another and larger sur- 

 face of lower constant temperature, the amount of heat which 

 is radiated to the latter per second will keep on increasing 

 until it is just equal to the amount produced per second by 

 the electric current. 



Between the inner heating wall and the outer cooling sur- 

 face we shall then realize the condition of steady flow, while 

 within the heating surface, thus held at a practically constant 

 temperature, we shall have exceedingly small variations in 

 the reading of the thermometer. 



An experimental test of the method was made as follows: — 



ice 



flaw 

 Cotton 



\y 



Ice 



Ice 



A cylindrical copper vessel C about 10 inches in diameter, 

 and having double walls, was mounted on three legs, two of 

 which (7, are showm in the figure. C can thus be packed 

 in a cvlindrical wall of ice. 



The^ whole is then set in a small dish-pan, which allow- 



