PRESSURE ON RESISTANCE OF METALS. 589 



and was supposed to be chemically pure in its original state. The 

 metal was first converted into the nitrate, and measurements made on 

 the polymorphic transitions under pressure. From the nitrate it was 

 converted to the iodide. These two conversions have been described 

 in a previous paper. ^° The iodide was then converted to the sulfate 

 by heating with c. p. sulfuric acid, and it was finally electrolyzed onto 

 a platinum electrode from aqueous solution of the sulfate. After all 

 these metamorphoses any impurity' of the original metal should have 

 been effectively removed. The purity of this electrolytic thallium 

 was tested by comparing its temperature coefficient of resistance with 

 untreated c. p. thallium from INIerck. The mean coefficient at 0° 

 from readings between 25° and 90° of electrolytic thallium was 0.005177 

 against 0.004898 of Merck's. The electrolytic is therefore appreciably 

 purer. 



Thallium was formed into wire about 0.013 inch diameter by cold 

 extrusion. It was wound loosely on a bone core and connections 

 made by soldering with "fine" solder (2 parts tin to 1 part lead). 

 Its initial resistance was 9.47 ohms at 0°. Thallium becomes coated 

 rather rapidly in the air with brownish oxide. The layer of oxide in 

 time becomes so deep as to very appreciably increase the resistance of 

 the wire. There is also a slow formation of oxide on standing in the 

 kerosene transmitting the pressure. The rate of formation of oxide 

 is not great enough to introduce appreciable error during a single run; 

 at 100° and 75°, where the rate of formation is most rapid, there was 

 no perceptible permanent change of zero after the runs. When the 

 apparatus stands over night, however, between runs, there is sufficient 

 formation to introduce appreciable error into the temperature coeffi- 

 cient determined from successive zeroes. The runs at 25° and 0° 

 were made 9 days after those at 50°, 75°, and 100°, and there was a 

 break in the resistance of 5% in this interval due to oxidation. For 

 this reason the relation between temperature and resistance has been 

 taken as linear in the table and the value chosen for the mean coeffi- 

 cient is that found from the comparison of electrolytic with Merck's 

 thallium. 



The wire was seasoned before measurements by subjecting to 9000 

 kg. at 95° and 12000 at 50°. 



The smoothed results are shown in Table III and the experimental 

 values in Figure 5. The maximum deviation of any point from the 

 smooth curve is 0.7% of the total pressure effect, and the average 



10 P. W. Bridgman, Proc. Amer. Acad. 51, 593 (1916), and 52, 151 (1916). 



