158 Experiments on Coated Plates 



this method must have been perfectly effectual, as it would be necessary 

 for the electricity to spread itself not only on the perpendicular surface 

 ef, but also to some distance on the horizontal surface fg, before the 

 quantity of redundant fluid lodged on the surface of the cement could 

 bear any sensible proportion to that in the coating CD. 



304] The result was that in dry weather the electricity seemed to 

 spread as fast on those plates which were covered with cement as on the 

 others, but in damp weather not so fast, the difference between dry and 

 damp weather being less in those plates which were covered with cement 

 than the others; and besides that there seemed as much difference between 

 the swiftness with which it spread on the surface of the Nuremberg and 

 English plates after they were covered with cement as before, which shews 

 plainly that the electricity spread between the cement and the glass, and 

 not on the surface or through the substance of the cement. It could not 

 be owing, I think, to its passing through the substance of the glass, for 

 if it was, there would hardly be much difference in the uncoated plates 

 between damp and dry weather, whereas, in reality, there was a very 

 great one. 



I also tried what effect varnishing the glass plates would have, but I 

 did not find that it did better, if as well, as covering them with cement. 



305] As there seemed, therefore, to be very little advantage in covering 

 the plates with cement or varnishing them, and as it was attended with 

 a good deal of trouble, I did not make use of those methods, but trusted 

 only to letting the wires down and up pretty quick, so as to allow very 

 little time for the electricity to spread on the surface of the plates, and this 

 I have reason to think was sufficiently effectual, as I never found much 

 difference in the divergence of the pith balls, whether the wires were let 

 down and up almost as quick as I could, or whether they were suffered 

 to rest a second or two at bottom. 



306] As the wire Cc is suffered to rest so short a time on Aa and Bb, 

 it is plain that the lower coatings of the trial plate and plate to be tried 

 must have a very free communication with the ground and the outside 

 coating of the jars, or else there would not be time for them to receive 

 their full charge. I accordingly took care that the wires which made the 

 communication should be clean and should touch each other in as broad 

 a surface as I could conveniently. As for the method I took to have a 

 ready communication with the ground, it is described in [Art. 258]. 



307] Besides this gradual spreading of the electricity on the surface 

 of the glass, there is another sort which is of much worse consequence, 

 as I know no method of guarding against it, namely, the electricity always 

 spreads instantaneously on the surface of the glass to a small distance 

 from the edge of the coating, on the same principle as it flies through the 



