PHYSICS: P, W, BRIDGMAN 
353 
instrumental error in the readings would have been 0.04%. The 
agreement is therefore as close as could be expected. At pressures 
lower than 12,000 the agreement of the readings was also always within 
possible instrumental errors. 
The temperature coefficients were compared by comparing the re- 
sistances at atmospheric pressure at 25° and 85°, while still wound on 
the same bone core as was used for the pressure measurements. Over 
this range the temperature coefficients differed by 0.06% of themselves. 
This is slightly greater than the possible instrumental error, but is 
less than possible discrepancies due to differences of handling which 
I have previously found in different lengths from the same sample of 
ordinary lead. 
The specific resistances were compared by comparing the actual 
resistances of the samples already measured. Correction for in- 
equalities of dimensions was made by measuring the length of each 
sample and its weight, and from these computing the average cross sec- 
tion assuming that the densities were directly proportional to the 
atomic weights.^ In order to measure the length, the wire had to be 
cut from the core, unwound, and straightened. Because of the me- 
chanical softness of lead, this is a difficult operation without error, so 
that the comparison of specific resistance is less accurate than of pres- 
sure or temperature coefficient. Two independent comparisons of 
specific resistance were made. The discrepancies of these two compari- 
sons differed in sign; the average of the two comparisons showed an 
agreement of specific resistance of 0.06%. 
Conclusion. — These measurements establish that any difference be- 
tween the pressure coefficient of resistance, temperature coefficient 
and specific resistance is at least many fold less than the difference 
of the atomic weights. It seems indicated with a high degree of proba- 
bility that the same conclusion will be found also to apply to the com- 
pressibility and thermal expansion. The results fortify the point of 
view embodied in recent theories of electrical resistance that the proc- 
esses involved in electrical conduction take place in the outer part of 
the atomic structure. 
1 Richards, T. W., and Wadsworth, C, 3rd, /. Amer. Chem. Soc, Easton, Pa., 38, 1916, 
(221-227 and 1658-1660). 
2 Richards, T. W., Year Book, Carnegie Inst., Washington, 16, 1917, (299-300). 
8 Aronberg, L., Astroph. J., Chicago, 47, 1918, (96-101). 
< Richards, T. W., and Wadsworth, C, 3rd, /. Amer. Chem. Soc, 38, 1916, (2613-2622). 
5 Bridgman, P. W., Proc. Amer. Acad., Boston, 49, 1914, (627-643). 
6 Bridgman, P. W., Ihid., 52, 1917, (573-646). 
