[coKER A m'kergow] THERMAL CHANGE TO TENSION 



comparatively large tension bars, using a modem testing machine for 

 loading the specimen. 



The most convenient means of observing the change of temperature 

 of a thermopile attached to a specimen under stress is a D'Arsonval 

 galvanometer, the field of which is very uniform in the neighbourhood 

 of large masses of feebly magnetized iron, such as are usually present 

 in a testing laboratory, and if care is taken to prevent the short cir- 

 cuiting of the thermopile in contact with the specimen by a thin sheet 

 of tissue paper, the changes in temperature due to the stress can be 

 easily followed and comparative readings obtained. The readings are, 

 however, not a faithful record unless certain precautions are observed, 

 for the moving coil of a D'Arsonval galvanometer docs not take up a 

 new equilibrium position at once, on account of its inertia and the lag 

 of the pile, and therefore any lag of this kind will cause an error in the 

 observations. It is always possible, however, to time the rate of loading 

 so that the reading of the galvanometer is a maximum for the stress 

 at that instant, and, as an example of the ease with which this can be 

 done, the following results may be quoted of a tension specimen 2.00 

 inches by 0.375 inches in section. The load was applied as uniformly 

 as possible, commencing with an initial load of 500 pounds and ending 

 with a load of 16,000 pounds. In this way the following observations 

 were made: — 



TABLE I. 



Resistance of pile = 5'55 ohms. 

 Resistance of galvanometer = 5'2 ohms. 

 Scale distance = 5'.. 10". 



Resistance of leads = 052 ohms. 

 One division on scale = 05 inches. 



