352 
PHYSICS: P. W. BRIDGMAN 
fied ordinary lead. The radio-active lead was from Australian carno- 
tite, and showed an atomic weight of 206.34, which is therefore 0.41% 
lower than that of ordinary lead. The theoretical and experimental 
value for the atomic weight of the pure end product of the disintegra- 
tion of uranium is 206.08, so that this sample was probably composed 
of 76% pure isotope and 24% ordinary lead. The radio-active lead 
contained not over 5 parts in 100,000 of impurity, mostly silver, and 
the ordinary lead was a trifle less pure, showing also a trace of copper. 
For the experiments both these samples were formed into wire 0.035 
cm. in diameter by cold extrusion through a steel die. The samples 
were cast into ingots ready for extrusion by Professor Richards, who 
melted them in hydrogen and then continued the fusion for ten minutes 
in vacuum. 
The measurements recorded here are comparisons of the pressure 
coefficient of electrical resistance, temperature coefficient of resist- 
ance, and specific resistance. The comparison of pressure coefficient 
of electrical resistance was made with more accuracy than the other 
measurements because a specially adapted apparatus designed for 
another purpose was available. 
In order to compare the pressure coefficients, approximately equal 
lengths of the two varieties of lead were wound non-inductively on 
either end of a bone core, which was placed in the pressure chamber. 
The two terminals of each wire were soldered to independent leads 
which were brought thrcugh the walls of the pressure chamber through 
an insulating plug of a design essentially like that previously described,^ 
except that there were three, instead of one, insulated stems through 
the plug. The two wires were made the two extension coils of a Carey 
Foster bridge. In this way the difference of the pressure effects on 
the two coils could be measured. The absolute value of the pressure 
effect on ordinary lead had been previously determined with sufficient 
accuracy.® The apparatus for producing pressure was the same as that 
previously described.^ 
Readings were made to 12,000 kgm./cm.^ at 1000 kgm. intervals, 
with increasing and decreasing pressure, and at two temperatures, 25° 
and 85°. An independent set of readings was made to determine the 
effect of pressure on the resistance of the leads, which turned out to be 
almost negligible. At 25° the decreases of resistance of the two kinds 
of lead under 12,000 kgm. were the same within 0.02% of the total 
decrease, and at 85° within 0.03%. Assuming that the possible error 
in reading the slider settings of the Carey Foster bridge was 0.1 mm. and 
that the errors conspired in the most unfavorable way, the possible 
