RICHARDS AND MARK. — EXPANSION OF GASES. 421 



The electromotive force and strength of the indicating current should be 

 very small, in order to avoid the occurrence of sparks. This point a 

 fixes both the volume and the level of the mercury. 



After the position of contact with a has been fixed, the pressure in the 

 opposite arm C is adjusted by changes in the amount of mercury in the 

 insulated reservoir R, so that contact is just made with the second point 

 a' also. The reservoir R is the outside reservoir referred to above, and 

 consists of a glass bottle of one and one-half litres capacity, packed 

 in cotton and resting inside a wooden box. The time between the final 

 adjustment of pressure within this reservoir and the reading of this 

 pressure is so short (from one to two minutes) , that with this amount 

 of packing, perceptible change in pressure due to change in temperature 

 does not take place. The pressure of the gas in A is the same as the pres- 

 sure in C and in the connecting reservoir R, plus or minus the difference 

 in height between the points a and a'. Before proceeding, the adjust- 

 ment at these two points is made again several times, in order that each 

 may be certainly established. 



The pressure in C and in R is now measured with great accuracy 

 by means of the Lord Rayleigh pressure gauge, shown in the left part 

 of the figure. Here contact is made with the lower point I of the iron 

 rod F. The observation is made through a magnifying glass perma- 

 nently fixed in position. The space // is exhausted by a Sprengel 

 pump and the vacuum obtained is measured by a McLeod gauge G. The 

 pressure in the tube C and reservoir R is thus measured by the height 

 of the mercury meniscus m above the point I. . This is easily determined 

 by measuring through a telescope the distance between point n and the 

 meniscus m, since the distance from the point I to the point n of the 

 iron rod F can be directly measured. This reading completes the per- 

 formance at the lower temperature. 



The ice in the bath B is then replaced by about ten kilograms of 

 sodic sulphate at its transition temperature, and a similar setting is 

 made, using the points b and V instead of a and a'. Since the tubes 

 are essentially alike at a, a', b, and V, and since the volumes are so 

 arranged that there is very little change in pressure upon change in 

 temperature of the bath, the conditions of measurement of pressure are 

 always the same, and the measurements may be made very accurately. 

 This uniformity of conditions eliminates constant errors, such as those 

 due to capillarity and a possible refraction of the glass. 



