Electrical Resistance of Glass

Electric Resistance of Glass.

Messrs. T. Gray, A. Gray, and J. J. Dobbie, of Glasgow University, have been investigating the electrical resistance of various kinds of flint glass, as made by different makers, with a view to find out the relations existing between the resistance and the chemical constitution of the glass. The tests were made by immersing flasks of the glass in mercury and filling up the interior with mercury. The mercury formed electrodes to convey the current from 120 Daniell cells through the glass; and the resistance was measured by a high resistance astatic galvanometer. They found that the resistance of the glass increased with the percentage of lead in the glass, and also with the density, but diminished as the percentage of alkali in the glass increased. The best specimen tried contained over 40 per cent. of lead oxide, and had a density of 3.141. Its specific resistance at a temperature of 130 deg. Cent., was 8400 X 1010 ohms between two opposite faces of a centimeter cube. The experiments show that the resistance of glass is halved by an increase of about 8% deg. Cent. This rule holds for a considerable range of temperature, hence the approximate resistance of glass at a certain temperature may be estimated. In taking the temperature observations, the glass flasks were heated by means of a sand bath.