298 



BRIDGMAN. 



thermal e.m.f. of the other metals is given. Experimentally, the 

 comparisons were all made against pieces of copper, cut from the 

 same spool, and the figures given were obtained by subtraction. It 

 is assumed here, from the work of previous observers, that the Thom- 

 son heat of this lead was zero; no direct examination of this question 

 was made for these experiments, as it would have required extensive 

 measurements with special apparatus. 



As a preliminary to the pressure measurements, one previous 

 application of 12000 kg. was made at room temperature to season. 



0° 



10° 



20" 30° 40° 50° 60° 70° 

 Temperature 

 Lead 



80° 90° 100° 



Figure 8. Lead. Thermal E.M.F. of a couple composed of one branch 

 of uncompressed metal, the other compressed to the pressure in kg./cm.^ 

 indicated on the curves, the junctions being at 0°C and the temperature 

 plotted as abscissae. 



The pressure measurements were very satisfactory, being much like 

 those on Thallium, although not quite so regular. The maximum 

 effect, which is positive, is 6 X 10"^ volts against 50 X 10"^ volts for 

 Tl. This doubtless accounts for the somewhat greater proportional 

 irregularity. The observed e.m.f's. are sensibly linear against pres- 

 sure at constant temperature. Such linearity is surprising in so soft 

 a metal as lead. The resistance curves against pressure were dis- 

 tinctly not linear. There was no appreciable change of zero at 

 any temperature, which again is most gratifying for so soft a metal. 



