259 
1909-10.] On an Electrically Controlled Thermostat. 
be avoided with a current which is strong enough to give a distinct 
minimum in the telephone. (2) The “capacity” of the cell must be high 
in order to prevent polarisation. (3) The cross-section must not be too 
large, or the cell becomes unwieldy. 
A tube 6 cm. in length and T5 mm. in diameter was found to be suitable 
for concentrated sulphuric acid. 
Sulphuric acid was chosen as, on the whole, the most suitable electrolyte 
with which to test the degree of accuracy attainable with highly conducting 
solutions. Hydrochloric acid has a higher conductivity than sulphuric 
acid, but at high concentration its volatility causes difficulties which affect 
the maintenance and determination of the concentration rather than the 
determination of the conductivity. 
In fig. 4, A, A are electrodes of thick platinum wire wound in the form 
of a hollow cone. This form of electrode gives a uniform distribution of 
the lines of flow over the whole surface. 
The ends of the electrodes are fused through the glass, and project into 
the mercury-cups B, B. The mercury and the platinum connecting wires 
(C, C) are kept in position by means of the layers of solid paraffin (D, D) so 
that the cell can be inverted without loss of mercury. The free ends of the 
wires dip into mercury-cups, which are connected by means of stout copper 
wires to the terminals of the Kohlrausch bridge. The resistance of the 
connections is determined by filling the cell with mercury and measuring 
the resistance. 
The method of procedure with this cell is as follows : — 
The cell is first cleaned with chromic acid mixture and washed with 
water. It is then dried in a current of air and weighed. The stopper F is 
then temporarily removed, a quantity of solution of known concentration 
introduced at G, the stopper replaced, and the cell weighed again. 
The stopper is again temporarily removed. A piece of rubber tubing, 
provided with a clip and guarded by an air-purifying tube, is now attached 
to the cell at E. By blowing air through this tube the solution can be 
stirred, and different portions of it brought between the electrodes. The 
level of the solution is fixed above A by replacing the stopper. The cell is 
finally immersed in the thermostat to the level of the dotted line, and the 
conductivity determined. The cell is re- weighed again to show that there 
has been no increase in weight during the determination. 
The solution adhering to the sides of the wider tube G above the 
capillary H is then washed into the cell with pure distilled water. The 
inside of the tube G is carefully dried with filter-paper, and the cell with 
solution is weighed again. The liquid in the cell is stirred and mixed as 
