THE MEASUREMENT OF STRESS BY THERMAL METHODS. 239 
In making observations with a galvanometer provided with a moving coil of con- 
siderable weight, such as that of the D’Arsonval type, the indications may not be a 
faithful record unless certain precautions are observed, for the galvanometer does not 
take up its position of equilibrium at once, and therefore any error due to this lag will 
make a considerable difference in the results. It was found experimentally that the 
time rate of loading could be so determined by trial that the reading of the galvanometer 
was very approximately a maximum for the stress at any given instant, and by making 
special experiments for each bar, the rate at which the loading ought to be applied 
could be easily determined. As an example of the accuracy with which the loading 
could be applied to keep step with the galvanometer, reference may be made to the 
results obtained from a steel bar of rectangular section, having a breadth of 0°86 inches 
and a thickness of 0°315 inches, which was loaded at a uniform rate until a maximum 
of 4000 pounds was reached, corresponding to a stress of 14,760 pounds per square 
inch. 
The readings obtained were as follows :— 
Aer VG 
Load, pounds. Time in seconds. | Observed Reading. | Corrected Reading. 
0) 0 0 0 
2000 25 4:0 4:23 
3000 45 70 (0 
4000 60 9°8 11:12 
4900 90 9-0 10°82 
4000 110 8:0 9°98 
4000 135 7:0 9:13 
4000 165 6:0 8:23 
4000 200 5:0 7°25 
4000 260 4-0 6°34 
4000 340 3:0 5°60 
4000 410 2:5 4°81 
Wotes.—Scale distance, 834 inches: 1 division =0°5 inches on scale. 
Resistance of thermopile and leads= 6°07 ohms, 
Resistance of moving coil of galvanometer= 52 ohms. 
Temperature 68° Fahr. 
From which it will be seen that as soon as the loading reached a maximum, the readings 
also attained a maximum, and then began to decrease. The ascending portion of the 
