THE ELECTRICAL RESISTANCE OF THIN LIQUID FILMS. 
471 
13 centims. apart, two fine holes were drilled in the tube, and pieces of platinum wire, 
the ends of which just projected beyond the inner surface of the glass, were cemented 
into these by sealing wax. The distance between these two points was measured by 
a cathetometer and the diameter was determined by weighing the quantity of mercury 
which occupied a portion of the tube a little longer than the distance between 
a and a!. 
If 
l— the distance between a and a '; 
r— the radius of the tube ; 
the resistance in ohms of the liquid between the two points ; 
p— the specific resistance, i.e., the resistance per cubic centimetre ; 
then 
-r. 7r? ’ 2 
p=Ex T 
7T7’2 
The value of the constant —— for the tube with which all the soap solutions were 
L 
measured was ’030001. The specific resistances were thus readily obtained by 
multiplying the observed resistances by ’03. 
The points a, a were sufficiently far from the bent up portions of the tube to 
ensure that the lines of flow between them would be parallel to the generating lines 
of the cylinder, and, as in the films, the platinum points a, a' would not sensibly 
interfere with this symmetrical flow. 
The two electrodes connecting with the battery consisted of flat spirals of platinum 
wire, the diameters of which were such as to allow them to move freely in the tubes 
b, b'. The object of using a spiral wire rather than a continuous disc was to facilitate 
the escape of the bubbles of gas formed by the decomposition of the liquid. 
The thermometers used had a range of 40° C, and were graduated to 0°’5, but 
could be read to 0°'l without difficulty. They were compared with a thermometer 
graduated to 0°'l which had been verified at Kew. Pieces of cork through which 
they passed supported them in their places in the tubes c, c. 
It is well known that the electrical resistance of a liquid depends in a very 
important degree upon its temperature ; and hence the necessity of knowing accurately 
the temperature of the soap solution at the time when its resistance was measured. 
To this end a water bath was contrived containing a chamber sufficiently large to hold 
the resistance tube, thermometers, &c. 
The water bath, which was made of tin plate, is represented in Plate 60 fig. 8, 
