On the Variation of the Electrical Resistance of Glass, fyc. 201 



trometer to earth ; while the other coating of the glass and the 

 remaining pair of quadrants were connected together but insulated. 

 The resistance was then calculated from the capacity of the glass 

 vessel and electrometer quadrant, and the rate of loss of charge. 



The conducting coatings for the glass were generally made by partly 

 filling the vessel to be experimented on with mercury, and then 

 immersing it in another vessel of mercury until the surface of the 

 mercury inside and outside the vessel was at the same level. In order 

 to avoid leakage over the sides of the vessel, it and the mercury were 

 made thoroughly dry by heating, and when sufficiently cooled, a 

 coating of paraffin was run over the surface of the mercury and the 

 vessel. Through this coating of paraffin a fine glass tube, well dried 

 and paraffined, was passed, thus furnishing at the same time a passage, 

 and more thorough insulation against surface leakage for the elec- 

 trode which made contact with the mercury. This explanation will be 

 more readily understood by the aid of fig. 1, which shows the arrange- 

 ment for measuring the resistance of a glass globe, the galvanometer, 

 battery, and key being shown symbolically. In the figure SM repre- 



Fig. 1. 



sents the surface of the mercury, Sp the surface of the paraffin, and t 

 the fine tube through which the electrode, I, passes. The tube, t, and 

 the neck of the globe were in such a case coated with paraffin. 



The precautions against leakage here described are more necessary 

 when the resistance at ordinary temperature is to be measured, but 

 even in other cases it was found advisable to begin with this, and 

 simply allow the paraffin to evaporate at high temperatures. The sur- 

 face of the hot glass remained afterwards perfectly dry. 



Sulphuric acid was sometimes used instead of mercury, and answers 

 perfectly if the temperature does not require to be high. If, however, 



p 2 



