244 PROCEEDINGS OF THE AMERICAN ACADEMY. 



long intervals of time. To test the effect of small metallic impurities, 

 two experiments were made on pure mercury contaminated with 

 known quantities of foreign metal, in the one case 0.1 per cent of zinc, 

 and in the other 0.1 per cent of lead. This is a very large quantity of 

 impurity, much larger than can possibly occur in practice. On stand- 

 ing a short while in the air, the surface of the mercury becomes posi- 

 tively filthy with oxides. The effect of 0.1 per cent zinc is to decrease 

 the resistance by about 1.4 per cent, but the pressure coefficient of 

 resistance by about 5 per cent. Furthermore, the departure from the 

 linear relation between total change of resistance and pressure is less 

 than for pure mercury, being 3 per cent less at 6500 kgm. The results 

 with the lead were not so satisfactory as those with the zinc. It was 

 pretty certain, however, that the effect of the lead is less on the total 

 resistance and greater on the pressure coefficient. 



The formulas given above connect the change of resistance of mer- 

 cury in a capillary of specified glass with the pressure, and are all 

 that is required for use with a secondary standard of pressure. The 

 observed change of resistance, however, is due to a combination of 

 two unrelated effects; the change of dimensions of the glass, and the 

 changed specific resistance of mercury. The results given above will 

 not possess theoretical value, therefore, until the two effects are sepa- 

 rated. In the following an experimental determination of these two 

 effects is given. 



We may distinguish two specific resistances of mercury, both of 

 which are altered by pressure. The first may be called the specific 

 volume resistance, and is the resistance of a body of mercury of in- 

 variable form, but of mass variable with the pressure. The second 

 may be called the specific mass resistance of mercury, and is the 

 specific volume resistance multiplied by the ratio of the masses within 

 the given surface at the variable and standard pressure, i. e., the 

 density. The specific mass resistance seeks to correct for the increased 

 conductivity to be expected at any pressure because of the increased 

 number of conducting particles in a given volume. In order to de- 

 termine the specific volume resistance, the above results have to be 

 corrected for the compressibility of the glass envelope; to determine 

 the mass resistance, an additional correction must be applied for the 

 compressibility • of the mercury. These compressibilities are deter- 

 mined in another paper, to which reference must be made for the 

 methods used. Only the results there found will be used here. It 

 was found that for Jena glass No. 3880 a, k = 2.17 X 10~ 6 , and that 

 the change of volume of mercury is connected with pressure by the 

 relation 



