Prof. Buff on the Conductivity of Heated Glass fw Electricity, 1 7 



^ Temperature or the glass . ' 



.in Centigrade degrees. Resistance. 



V 200 2582-0. r 



!, 250 158-3.r 



300 leS.r 



^......u... 350 11-8. r 



400 8-4. r M 



Electric Polarization of Glass. 



It has been already mentioned that the glass covered on both 

 sides by mercury, after it has permitted of the passage of ari 

 electric current for some time, exhibits an independent electro- 

 motive force which acts in opposition to that of the current, and 

 which may be exhibited both by the galvanometer and the gold- 

 leaf electrometer. It was natural to regard this phsenomenon as a 

 kind of charge similar to that of the condenser. In harmony 

 with this view was the fact, that the effect soon disappeared on 

 connecting the two mercury coatings with a good conductor. A 

 closer examination of the subject, however, does not appear to 

 justify this assumption. 



When the two coatings of a condensing apparatus are con- 

 nected with the poles of a voltaic battery, it is known that a 

 momentary contact is sufficient to obtain the maximum charge, 

 and this maximum increases almost in the same proportion as 

 the strength of the battery. The inner and outer coatings of 

 the glass tube behaved exactly similar as long as low tempera- 

 tures were made use of; when, however, the temperatures were 

 high, very remarkable divergences appeared, as the following 

 experiments will show. 



One of the mercury coatings was connected with the positive, 

 and the other with the negative pole of a Bunsen^s battery of 

 three elements. At a temperature of 16° C. a charge was ob- 

 tained, which, after the exclusion of the battery, produced a 

 divergence of one inch of the gold-leaf electrometer. This cor- 

 responded to the maximum action of the three elements. At a 

 temperature of 50° C. this divergence amounted under the same 

 circumstances to 4 lines, at 100° to only 1 line, and at 300° 

 sunk to less than half a line, although a small divergence was 

 still distinctly perceptible. Notwithstanding this great diminu- 

 tion in the strength of the charge in the case of the heated glass, 

 a brief contact with the poles of the battery was by no means 

 sufficient to produce the maximum charge ; for this purpose, on 

 the contrary, a contact of several minutes was necessary. When 

 the charge was once imparted, it was much more persistent than 

 that obtained at ordinary temperatures by momentary contact. 



Phil. May. S. 4. Vol. 8. No. 49. July 1854. C 



