PRESSURE COEFFICIENT OF RESISTANCE. 155 



measurement on two different samples. The maximum deviation 

 from linearity of this new pure sample is not as simple as that of the 

 less pure one; the former decreased linearly with rising temperature, 

 whereas this new sample shows a minimum. 



The temperature behavior at atmospheric pressure is worth 

 comment. In the first place, the strict linearity with temperature is 

 not usual. In the second place, the high value of the temperature 

 coefficient of this piece as compared with the former sample, or the 

 values of other observers, is to be noticed. The establishment of this 

 high value removes aluminum from an apparently unique position 

 with regard to its temperature coefficient. It is well known that 

 practically all metals have a higher temperature coefficient in a condi- 

 tion of higher purity. Aluminum appeared to be the only exception, 

 since the published values for samples of increasing purity have ap- 

 parently become smaller with increasing purity. 3 But the purest of 

 these previous samples had about 0.1% impurity. It is now seen that 

 the apparent anomaly disappears in the region beyond 0.1%, the 

 initial trend being reversed, and at sufficiently high purities the 

 temperature coefficient increases with increasing purity, as it does for 

 all other metals, and furthermore, the coefficient of the purest metal is 

 higher than that of any of the impurer specimens. 



Nickel. This was of exceptionally high purity, and I owe it to the 

 kindness of Mr. I. B. Smith, of the Research Laboratory of Leeds and 

 Northrup Co. I have already published data for the effect of tension 

 on resistance, 4 and pressure on thermal conductivity 5 of this same 

 nickel. Data for the compressibility are to be published shortly in 

 These Proceedings. Hitherto it has not been possible to obtain in this 

 country nickel of purity higher than that corresponding to a tempera- 

 ture coefficient of resistance of 0.0049. The coefficient of this was 

 0.00634, and is higher than any other published value except 0.00683 by 

 Niccolai. 6 Except for the value of the temperature coefficient, I have 

 no chemical analysis to indicate the purity. 



The material was drawn by Leeds and Northrup to wire 0.0127 cm. 

 in diameter, was annealed to redness, and then was double covered 

 with silk insulation by the New England Electrical Works. It was 

 wound non-inductively on a small glass core, seasoned at 135° for six 

 hours, and subjected to three preliminary applications of 12000 kg. 

 The initial resistance at 0° was about 115 ohms. The measurements 

 were made on the Carey Foster bridge regularly used in measuring the 

 changes of resistance of samples with high resistance. 



The regular series of pressure measurements was made, at 0°, 25°, 



