PRESSURE COEFFICIENT OF RESISTANCE. 



153 



Aluminum. This was a specimen of quite unusual purity, of the 

 following analysis: Si 0.014, Fe 0.007, Cu 0.003, Al (by diff.) 99.970. 

 I extruded it from a diameter of 1.27 cm. to 0.081 cm., from this 

 diameter drew it through steel dies to a diameter of 0.051 cm., annealed 

 it at a temperature of 350°, wound it non-inductively on one of the 

 bone cores used in previous work, made connections to the current 

 and potential leads with spring clips at the two ends, and made the 

 measurements in the regular way on the potentiometer. The length 

 was such as to give an initial resistance of about 0.2 ohms. 



The regular procedure in making the measurements was followed. 

 Several preliminary seasoning applications of pressure were made, 

 and runs were made at 0°, 30°, 50°, and 95°. The run at 75° failed 

 because of a minor accident. At 0° the average arithmetical deviation 

 of the observed points from a line (no discards) w T as 0.23% of the 

 maximum pressure effect, at 30° (one discard) 0.13%, at 50° (two dis- 

 cards) 0.6%, and at 95° (one discard) 0.17%. The deviation from 

 linearity was symmetrical about the mean pressure within the limits 

 of error, so that the changes of resistance can be represented by a 

 second degree expression in the pressure. In addition to these 

 pressure measurements, the temperature coefficient at atmospheric 

 pressure was determined by measurements at 0°, 30°, 50°, 75°, and 95° 

 in this order, and then the measurements at 50° and 0° were repeated. 

 The repeated readings agreed with the original ones within the limits 

 of setting the slider of the bridge wire, showing no temperature 

 hysteresis. 



TABLE I. 



Aluminum. 



