﻿312 Mr. Baly and Dr. Ramsay on the Relations of Pressure, 



quantities of mercury were run in from a fine capillary tube 

 which was pushed up (0) and (D). From the numbers thus 

 obtained the volumes represented by each reading-point could 

 easily be calculated. Lastly, the total volume of the gauge 

 from the top of (D) to the trap (E) was found by weighing 

 the gauge empty and then filled with mercury. From the 

 data thus obtained a table was constructed giving the capillary 

 corrections and volumes in cubic centimetres for each of the 

 fifty reading-points, due allowance being made for the 

 meniscus. A column was also added of the values of R for 

 each graduation, R being the ratio of the total volume of the 



y 

 gauge to the small volume at each point, or — . This ratio is 



used in calculating the vacuum in millimetres in the appa- 

 ratus from a reading of the gauge. McLeod has shown that 

 if a pressure p be read, and R be the ratio at a point on the 

 gauge, then the vacuum will be found from the expression 



4j j or, if a second approximation be required, from 



R * 



The method of calculating the numbers read on the gauges 

 was to multiply each pressure by each volume. 



The following table shows the accuracy to be expected : — 



v'. 



c. 



v". 



P'- 



p in 

 mm. 



v in 

 c.c. 



pv. 



120 



7-2 



127-2 



-11-0 



116-2 



•129 



14-99 



100 



7-1 



107-1 



+31-9 



1390 



•107 



1493 



80 



7-1 



87-1 



+86-9 



174-0 



•085 



14-91 



70 



71 



771 



+1226 



199-7 



•074 



14-94 



60 



71 



67-1 



+166-5 



233-6 



•064 



14-93 



In the first column v' are given the reading-points on the 

 gauge volume-tube. In the second column c are given the 

 capillary corrections ; under v" the result of applying these 

 corrections (v"=v'±e). Under p' the readings of pressure 

 on the gauge, p the actual pressure exerted by the mercury 

 on the measured gas p = v" +p', plus or minus according to 

 whether the reading ol ' p r is above or below the zero-point on 



