THE ELECTRICAL RESISTANCE OF THIN LIQUID FILMS. 
463 
caustic potash and pumice soaked in strong sulphuric acid respectively, and then over 
a surface of liquide glycerique contained in a horizontal tube about 2 centims. in 
diameter. When a constant state was required, the first film made was never used ; 
it was left until the hygrometer and thermometer had become steady, was then broken 
without opening the case by giving it a sharp blow with the needles, and replaced by 
another. If during this operation the hygrometric reading altered by more than one 
or two tenths of a division, the second film was also rejected, and so on. In general, 
the films used had been formed without any appreciable change in the hygrometric 
state. Minute and even excessive as these precautions may appear, we found, as will 
be shown hereafter, that they were barely sufficient for our purpose. 
The principal use of the chamber P was to enable us to keep the liquid used in 
making the films unchanged, when for special purposes we wished to charge the inner 
case with aqueous vapour at a greater or less tension than that proper to the liquide 
glycerique. 
Insulation tests— As before stated, the ebonite pillar carrying the three needles was 
protected from the spray of the bursting films by a glass shield, with holes in it for 
the needles to pass through. As, however, the precautions just described filled the 
case with large quantities of vapour, it was possible that in the course of a few hours 
a deposit of moisture might be formed on the ebonite pillar, or upon the glass sides of 
the apparatus, of such conducting power as seriously to interfere with the accuracy of 
the experiments. To guard against error from this source, the insulation of the 
needles from each other, and of the needles and the upper and lower cups from the 
case, was tested at the end of each set of observations. The observations were 
regarded as satisfactory so long as the greatest resistance measured did not exceed 
0 - 3 per cent, of the insulation resistance. When, as happened on one or two occasions, 
it greatly exceeded this limit, the preceding observations were rejected. 
VII.— Description of method employed for measuring the electrical resistance of 
the films. 
The battery consisted of 4 Leclanche cells of the ordinary pattern. A simple 
circuit was formed, comprising the battery, three boxes of resistance coils, and the film 
to be experimented upon. The resistances consisted of (1) a box of coils ranging from 
1 to 10,000 ohms, (2) ten coils each of 10,000 ohms, and (3) ten coils each of 100,000 
ohms. In the case of most of the films recently observed, the resistance in the boxes 
was either 500,000 or 1,000,000 ohms, and two binding screws, one on each side of 
this resistance, could be connected by carefully insulated wires with the electrometer. 
The current always passed through the film from the top to the bottom. 
The Electrometer. —The electrometer used is one of Sir William Thomson’s quadrant 
instruments, made by W hite ; it retains its charge remarkably well, a few turns of the 
replenisher being sufficient to bring the hair up midway between the two dots after 
