ELECTRICITY. 483 



that the form of the two conductors may be such, as to 

 bring every part of each under their mutual influence, 

 simple sheets of tin foil are used, pasted, one on each 

 side of the plate of glass. Now if one of these sides is 

 connected with the electrical machine, and the other 

 with the ground, both will become, on principles already 

 described, highly charged; the one side by direct 

 communication and the other by induction, as explained 

 on page 477. The two sides will however be charged 

 with opposite electricities. 



A glass plate, coated in this way, on the two sides, 

 would not be a very convenient apparatus. The same 

 arrangement is however effected by means of a jar, which 

 is coated within and without. From the inside there 

 arises a wire with a knob upon the top. This knob is 

 brought into contact, when the machine is in operation, 

 with the prime conductor, and thus the inside of the jar 

 is positively electrified. By the influence of the electrici- 

 ty thus conveyed into the inside of the jar, acting through 

 the glass, the outside is thrown into the opposite state, 

 and by forming a communication between the two, a 

 discharge results, which produces far more powerful ef- 

 fects than the prime conductor. 



It ought to be noticed, however, that the form of the 

 jar, and the manner in which it is coated, are of no 

 consequence. All that is essential is that there should 

 be two metallic surfaces, separated by glass. Sometimes 

 the jar is filled with shot. Sometimes coated with tin- 

 foil, sometimes with brass or iron filings, made to 

 adhere by gum water. The experiment will even suc- 

 ceed with a tumbler, half filled with water, for the inside 

 coating, and covered on the outside, as high as the sur- 

 face of the water, with wet paper. In all cases the up- 

 per part of the glass, which is not covered with the coat- 

 ing, should be kept perfectly dry and clean, lest by the 

 moisture accumulated there, there should be an acciden- 

 tal communication formed between the inside and out- 

 side of the jar. 



' It is hardly necessary to observe, that every species of 

 electrical machine will naturally require some particular 

 precaution ; but the following directions are more or less 



