46 BRIDGMAN. 



high value is in Hne with our other experience that a comparatively 

 soft metal has a Poisson's ratio near 0.5. 



Calcium. This material I owe to the kindness of the Research 

 Laboratory of the General Electric Company. It was from a different 

 batch than that whose pressure coefficient of resistance I have previ- 

 ously measured.'^ So far as any chemical analysis can detect, all the 

 calcium of the General Electric Co. contains no impurity, but in my 

 previous discussion I remarked on the fact that there w^as nevertheless 

 some difference between different batches. Some of the material can 

 be extruded easily, while the extrusion of other is difficult. The wire 

 which I previously used was extruded with difficulty, and was inclined 

 to be brittle. The present wire was extruded easily, and could be 

 readily bent into a comparatively short radius. The wire as supplied 

 by the General Electric Co. had been extruded to a diameter of about 

 0.055 inches. In order to better adapt it to the magnitude of the 

 tension which I could readily apply, I drew it down through steel dies 

 from this size to 0.030 inches, first scraping the surface bright under 

 oil. I did not attempt to anneal it after this drawing. Another 

 piece, whose behavior beyond the elastic limit was specially examined, 

 was drawn to 0.019 inches. The breaking load was at the rate of 

 1200 kg/cm^; it was the same for the two sizes of wire. The break 

 takes place with very little elongation or reduction of area. 



Measurements were made of the elastic rate of change of resistance 

 on two different samples. The range of tension was not more than one 

 fifth of the breaking load. The effect with this metal is large enough 

 to allow good readings. Within the limits of error the change of 

 resistance is linear with tension up to the stresses mentioned above. 

 Readings were made at eight or ten different loads. The maximum 

 departure of any single observation from a straight line was 5% of the 

 maximum change for one of the specimens, and 4% for the other.^ 

 The maximum change of resistance was 0.14% of the initial value. 



The tension coefficient of resistance was + 8.24 X 10"^ for one 

 specimen, and 8.50 X 10~^, tension in kg/cm-, for the other. Take as 

 the most probable value the mean + 8.37 X 10~^. 



Young's modulus was determined from the bending of two samples, 

 whose dimensions were of the same order as those of lithium. The 

 maximum load applied was 0.24 gm. Within the limits of error the 

 bending was proportional to the load. One specimen gave for 

 Young's modulus 2.080 X 10", and the other 2.065 X lO^i, in Abs. 



7 Reference 5, p. 91. 



