172 Experiments on Coated Plates 



340] In order to try the charge of what Jipinus* calls a plate of air, 

 I took two flat circular plates of brass, 8 inches in diameter and J thick, 

 and placed them on the bars Nn and Pp of the machine (Fig. 20), the two 

 plates being placed one over the other, and kept at a proper distance from 

 each other by three small supports of sealing-wax placed between them, 

 the supports being all of the same height, so that the plates were exactly 

 parallel to each other. Care was also taken to place the plates perpendicu- 

 larly over each other, or so that the line joining their centers should be 

 perpendicular to their planes. 



The lowermost plate communicated with the ground by the wire RS, 

 and the uppermost communicated with Mm by the wire V, just as was 

 done in trying the Leyden vials. 



I then found its charge, or the quantity of redundant fluid in the 

 uppermost plate, in the usual manner, by comparing it with the plate D, 

 and found it to be to that of D asf.... 



341] As I was desirous of trying larger plates than these, and was 

 unwilling to be at the trouble of getting brass plates made, I took two 

 pieces of plate-glass J n inches square, and coated each of them on one 

 side with a circular plate of tinfoil 11-5 inches in diameter, and placed 

 them on the machine as I did the brass plates in the former experiment, 

 with the tinfoil coatings turned towards each other, and kept at the 

 proper distance by supports of sealing-wax as before, care being taken 

 that the tinfoil coatings should be perpendicularly over each other. 



For the more easy making a communication between the circular 

 coating of the lower plate and the ground, and between that of the upper 

 plate and the wire Mm, I stuck a piece of tinfoil on the back of each plate, 

 communicating by a narrow slip of the same metal with the circular 

 coatings on the other side. 



I then tried the charge as before, the lower plate communicating with 

 the ground and the upper with the wire Mm. 



As glass does not conduct electricity, it is plain that the quantity of 

 electric fluid in the pieces of tinfoil will be just the same that it would be 

 if the glass was taken away, and the pieces of tinfoil kept at the same 

 distance as before. 



The distance of the two circular coatings of tinfoil was measured by 

 the same instrument with which I measured the thickness of the plates 

 of glass, and may be depended on to the loooth or at least to the 5ooth 

 part of an inch. 



* \Mirn. Berl. 1756, p. 119.] 



f The memoranda I took of that experiment are lost, but to the best of my 

 remembrance the result agreed very well with the following experiment. 



J [Art. 517.] 



[See Art. 459, "Bird's instrument," and "dividing machine," Art. 517. Also 

 594. 595-] 



