172 BRIDGMAN. 



being 0.45%. This is much less than for any of the pure metals, and 

 is seven times less than for manganin. It is possible to find other 

 nickel-chromium alloys that have an even smaller coefficient, but this 

 was so small that it was not necessary to go out of the way for other 

 material. 



Other corrections, not as important as the above, are for the temper- 

 ature coefficient of resistance of nichrome wire, and for the difference 

 of compressibility between the wire and iron. Both these corrections 

 were determined by direct experiment; the determination of the 

 compressibility will be described later. 



Although there are several corrections to be applied, it is a unique 

 advantage of this method that the corrections are very small, and that 

 the greater part by far of the measured effect is the final effect that is 

 wanted. The corrections just discussed for the change of resistance of 

 the nichrome wire, etc., did not attain as much as 2% of the measured 

 effect, and the correction for the external change of length of the 

 cylinder was only 1.5%. On the other hand it frequently happens 

 that the corrections in such methods as used by Richards or A. W. J. 

 may be two or three times as large as the final effect wanted. 



It is evident that the exact uniformity of cross section of the wire B 

 is of vital importance if the changes of resistance are to give slight 

 departures of compressibility from linearity -with pressure. To test 

 the uniformity of the wire a special arrangement was made consisting 

 of two knife edges a fixed distance apart, mounted to slide along the 

 wire, which was stretched along a meter stick. A constant current 

 was passed along the wire, and the difference of potential between the 

 two knife edges measured on the potentiometer. The wire was ex- 

 ceedingly uniform. In the first length tried, the extreme variation in 

 a length of 20 cm. was less than 1/3000. The variation in a length of a 

 millimeter or less was of course much smaller, and was obviously far 

 too small to introduce any appreciable error. 



The iron used for the measurement of linear compressibility was cut 

 from a bar of American ingot iron. This iron is very pure, containing 

 only 0.03% impurity, and was considerably purer than the piece of 

 boiler plate whose compressibility was previously measured. The 

 resistance under pressure of a piece cut from this same material has 

 been previously measured. 6 The dimensions of the compressibility 

 specimen were approximately 6 mm. in diameter and 30 cm. long. It 

 was annealed at a red heat after machining. 



The containing cylinder was especially made for this experiment 

 and was of chrome vanadium steel from the Halcomb Steel Co., sold 



