MY IK>T PLATmuM IN DIFFERENT OA8B. 27 



to the platinum there is no reason for taking one set of values rather than the other. 

 < >n these grounds the results at low pressures may he considered more reliuhle than 

 those at high onea 



8. Cnini>nnxon of !)<()'< rent 11 V/v.v. 



After the last section it is refreshing to find that the ionisation from hot wires is 

 capable in some ways of exhibiting a certain amount of constancy. The results given 

 on pp. 24 ami -J") show that two wires at any rate gave approximately the same amount 

 of ionisation, both positive and negative, per unit area with the same pressure of oxygen 

 at all temperatures from 700 C. to 1250 C. In the course of the investigation the 

 positive ionisation in oxygen, under similar conditions of pressure and temperature, 

 was measured for four different wires having different linear dimensions. It is 

 interesting to compare the leaks from these per unit area of surface when reduced, 

 by means of the results which have been obtained, to some standard pressure and 

 temperature. The pressure selected is 1'5 millima, and the leaks at this pressure are 

 given for two temperatures, viz., 770 C. and 880 C. The data will be found in the 

 following table (see next page). 



The wires numbered (1), (2), and (4) were each O'l millim. in diameter, while 

 No. (3) was 0'2 millim. in diameter. It will be noticed that, although the area of 

 the wires varied in the ratio of nearly one to four, and the different wires had been 

 very differently treated, yet the values of the leak per unit area, as shown by the 

 last two columns, are very nearly the same in every case. The values furnished 

 by the first wire on May 5 and July 14 are especially interesting, since they show 

 that, once the steady condition is attained, there is no further falling off due to 

 continued heating in oxygen at a red heat. This wire was heated for several hours 

 on most days between the two dates to a temperature varying between 750 C. and 

 1100C. As the observations recorded with wire No. 3 near the temperatures of 

 770 C. and 880 0. were a little irregular, the values were selected by drawing a 

 curve like fig. i) and finding the values of the leak at these temperatures from the 

 curve. In regard to this wire it is only fair to say that it was subsequently heated 

 strongly in hydrogen, and after that treatment was found to give a much smaller 

 leak (about one-third to one-fourth) than before. It seems probable, however, that 

 this is due to a permanent change produced by the hydrogen in the texture of the 

 metal surface. It has often been observed that the surface of platinum which has 

 been heated in hydrogen develops a roughened crystalline appearance. This change 

 does nut appear to be produced by heating in oxygen, at any rate at temperatures 

 IM-|O\V 1100 C. The alteration could not U> due, to an error in the temperature 

 brought alxmt by a change in the temperature coefficient of the resistance, as the 

 negative leak \\.-is not reduced in as great a ratio as the positive. The figures for 

 the last \\ire are of interest, as they were obtained in the tulie with water-cooled 

 walks and wore t lit- re fore free from any effect from the walls. 



2 



