NOTES AND COOLIDGE. — ELECTRICAL CONDUCTIVITY. 167 



bomb has not become completely full of liquid ; and secondly, it furnishes 

 a means of measuring the specific volume of the solutions. The first 

 provision is necessary since the bomb is designed to withstand the vapor 

 pressure, but not the fluid pressure of the liquid. A knowledge of the 

 specific volume is required in order to calculate the equivalent from the 

 observed conductivity. A measurement of the resistance between Lj 

 and L3, together with a measurement of that between Lx and Lo when 

 preceded by a calibration which may be made once for all, shows, 

 as will be explained more fully in Section VI., at any time after the 

 solution has expanded sufficiently to come into contact with the aux- 

 iliary electrode, just how high the liquid stands, and therefore how much 

 vapor space remains. 



The small platinum tube, T^, serves to exhaust the air from the bomb. 

 The method of doing this will be apparent from the diagram and the 

 following description. The hollow screw K is connected by means of 

 rubber tubing with a Richards water pump, and is at first raised so 

 that air can come out under the little steel bicycle ball which rests on 

 the upper end of the platinum tube. After the air is removed until 

 a pressure gauge shows a pressure within of about 2 cm., and while the 

 pump is still in operation, the part K is screwed down, thus forcing the 

 steel ball onto its seat and closing the end of the tube. 



The solution comes into contact with nothing but platinum, quartz- 

 crystal, and gold, except at the top of the narrow tube, Ti, where it 

 may touch the steel ball. The latter could be gold plated; but this 

 has proved unnecessary, since there is scarcely any circulation through 

 the narrow tube. 



The lower electrode, as well as the auxiliary electrode and its sur- 

 rounding tube, are well platinized. The body of the lining is not 

 platinized, since on account of its great surface this is not necessary. 



2. The Conductivity Measuring Apparatus. — The conductivity was 

 measured by the ordinary Kohlrausch-Wheatstone Bridge method, using 

 the induction coil and telephone. The slide wire was of platinum- 

 iridium ; it was one meter in length and 0.4 mm, in diameter. The 

 resistance coils, 2000 ohms in all (or 4000 ohms in a few measurements), 

 were of manganine. The whole conductivity apparatus was mounted on 

 a small portable table so that it could be moved about as the bomb was 

 changed from one heating bath to another. It was always kept at a 

 distance from the heaters. No temperature correction needed to be 

 applied to the resistance coils. Heavy flexible copper leads were used 

 up to within a few centimeters of the top of the heaters, where they were 



