ELECTRICAL RESIST.^NCE UNDER PRESSURE. 131 



method need not be described; they were sufficiently obvious. 

 Asymmetry in the specimen was eUminated by making two sets of 

 readings with the hot and cold ends reversed. The difference of 

 readings in the two positions was only 3 %. The temperature interval 

 was from 0° to 21°. The effect is very large. The thermal e.m.f. 

 against copper in this interval was at the rate of 0.000413 volts per 

 degree Centigrade, positive current flowing from copper to phos- 

 phorus at the hot junction. 



Iodine. The measurements on iodine were part of the systematic 

 attempt to measure the eft'ect of pressure on the resistance of all the 

 elements which could be handled with sufficient ease. The striking 

 effects found for black phosphorus, and the nearness of phosphorus 

 and iodine in the periodic table gave rise to the hope that a similar 

 effect might be found with iodine. 



The measurements proved of unexpected difficulty, because of 

 the readiness with which iodine dissolves in most of the liquids by 

 which pressure can be transmitted. It was found, for instance, that 

 iodine dissolves in kerosene, or petroleum ether, or glycerine. It was 

 a siu-prise to find that the solutions are fairly good conductors. Con- 

 siderable effort was spent in devising a suitable method of trans- 

 mitting pressure to the iodine and getting electrical connections into it, 

 but without much success. In the arrangement finally adopted, the 

 iodine was melted into an open glass cup, provided with two platinum 

 electrodes connected with wires sealed through the base. The cup 

 was placed in a second larger cup, and the wires led up between the 

 inner and the outer cup, and bent over the edge of the outer cup. 

 The upper part of the inner cup was filled with water to a sufficient 

 depth to completely cover the iodine. The outer cup was filled with 

 Nujol, which covered the iodine and water in the inner cup. Connec- 

 tions were made as usual to an insulating plug, this time one of the 

 old single-terminal plugs. The object of the double arrangement of 

 cups was to keep the iodine from contact with the oil, and to keep the 

 water from contact with the insulating plug and any part of the leads, 

 wdiich would otherwise have been short circuited. The arrangement 

 was not satisfactory, for the glass cracked around the platinum leads 

 under pressure, allowing a slight amount of iodine to go into solution 

 in the oil, and furthermore, because of unequal compressibility of the 

 glass and iodine, some water crept between the surface of the glass 

 and the iodine, thus making a short circuit possible. The iodine 

 further dissolved to some extent in the w^ater under pressure, and 

 from the water it again diffused into the surrounding oil, so that there 



