Spreading of charge on glass 157 



uncoated part of the glass into CD, and on the opposite side it will flow 

 into the uncoated part of the glass from cd. 



301] There is a great deal of difference in this respect between different 

 kinds of glass, as on some kinds it spreads many times faster than on 

 others. The glass on which it spreads the fastest of any I have tried is 

 a thin kind of plate-glass, of a greenish colour, much like that of crown- 

 glass, and which I have been told is brought from Nuremberg*. On the 

 English plate-glass it does not spread near so fast, but there is a great 

 deal of difference in that respect between different pieces. On the crown- 

 glass it spreads not so fast as on the Nuremberg, but I think faster than 

 on the generality of English plate-glass. On white glass I think it spreads 

 as slowly as any. 



302] The way in which I compared the velocity with which it spread 

 on different plates was as follows f. I took away the wire Ff (Fig. 20) 

 and placed the plate which I wanted to try where the plates L or I used 

 to be placed, the lower coating communicating as usual with Dd by the 

 wire 8, but the wire /J being drawn up by a silk string so as not to touch 

 the upper coating. The wire Cc is suffered to rest on Aa and the jars 

 electrified. When they are sufficiently charged /J is let down on the upper 

 coating, which instantly charges the plate to be tried, and immediately 

 the wire Gg is lifted up from Dd, but not high enough to touch A a. Con- 

 sequently, immediately after the plate is charged, the communication 

 between Dd and the ground is taken away, and consequently as fast as 

 any fluid flows from the uncoated part of the under surface of the glass 

 to the lower coating, some fluid will flow into Dd and overcharge it, and 

 consequently make the pith balls separate. 



303] In order to prevent, if possible, the ill effects proceeding from this 

 spreading of the electricity, I took some coated plates of glass, and covered 

 all the uncoated part with cement to the thickness of J or \ an inch, as 

 in Fig. 22, which represents a section passing through the middle of the 



Fig. 22. 



plate perpendicular to its plane, and in which the glass plate and coatings 

 are represented by the same letters as before, and the dotted lines re- 

 present the cement | . Thinking that it would be impossible for the 

 electricity to spread between the cement and the glass, in which case 



* [Art. 497.] t [ See Arts - 4 8 5' 4 86 - 4 8 7- Also Arts - 494 to 499.] 



t [Art. 484.] 



