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Proceedings of the Royal Society of Edinburgh. [Sess. 
described above, and the conductivity is again determined. The cell with 
solution is weighed again, and more water is added, and so, on till a series 
of readings of conductivity at different concentrations has been obtained. 
Weighings of the cell when empty, or the consecutive weighings with a 
given solution, never varied by more than one milligram, which is of the 
order of 70,000 the weight of the solution. 
The bridge used is of the large roller type described in Kohlrausch and 
Holborn’s Leitvermogen der Electrolyte, p. 42, and was made by Hartman 
and Braun. Calibrated by the method of Strouhal and Barus, no appreci- 
able errors were found. 
Extension resistances of 4*5 times the resistance of the bridge wire were 
connected to each end of the bridge. The resistances were found to be 
concordant among themselves. 
The source of current is a small induction coil actuated by two storage 
cells in series. A Fischer interruptor independently actuated by two 
other cells, and having a very high and steady note, is used to make and 
break the primary circuit. 
The strength of current going through the cell is regulated by a shunt 
put across the terminals of the secondary coil. 
The shunt should be varied at each determination to prove the absence 
of heating in the cell. When a reduction in the shunt produces no change 
in the reading, the heating effect may be considered to be eliminated. 
The introduction of a small condenser at some part of the circuit, as 
described by Kohlrausch and Maltby, was generally found to improve the 
minimum.* For the conductivity determinations recorded below, the 
sharpness of the minimum was not considered satisfactory unless a 
movement of the bridge wire through Tu ^ooo its l en gth to either side 
produced a distinctly perceptible increase of sound in the telephone. It 
was generally sharper. 
The sulphuric acid used had been re-distilled, and was specially pure. 
Twenty grammes of the acid evaporated in a platinum crucible gave no 
weighable residue. 
By Nessler’s test the acid was found to contain less than five parts per 
million of ammonia. It was free from nitrous, nitric, and sulphurous acids. 
A stock solution was prepared by mixing equal parts of this acid and 
pure water. This solution was preserved in a stoppered glass bottle which 
had contained concentrated sulphuric acid for several years. The stopper 
was provided with a cover, which was sealed with paraffin to prevent any 
* Wissenschaftliche Abhandlunqen der Physikalisch- Technischen Beichs Anstalt, Band iii. 
(1900), p. 165. 
