100 ELEMENTS OF STATIC ELECTRICITY. 



If the chain be removed entirely, only a very limited 

 charge can occur, derived from the material of the 

 machine itself. 



The limit of the charge is reached when its potential 

 energy, whether positive or negative, so far exceeds the 

 resistance of the air, that the loss of charge by convec- 

 tion, as explained on page 66, shall equal the energy 

 generated. When the atmosphere is damp, or its 

 electric potential low, this limit is soon reached; but 

 when dry, and at a high electric potential, a much 

 greater charge can take place. 



MACHINE DESCRIBED BY NOAD. The largest ma- 

 chine of this kind of which we have any record was 

 made some years ago for the Panopticon of Science in 

 London. According to Noad, it had a plate ten feet in 

 diameter, three pairs of rubbers, each three feet in 

 length, and a pear-shaped prime conductor, six feet 

 in length, and four feet in diameter at its widest 

 part. 



It was operated by steam power, and gave sparks 

 fifteen to eighteen inches in length; and charged to its 

 full capacity, in less than a minute, a Leyden battery 

 of thirty-six jars, having one hundred and eight square 

 feet of coated surface. 



MEASUREMENT or ENERGY. The amount of elec- 

 tricity which a well-constructed machine can generate 

 is in proportion to the surface area of the plate, which 

 may be increased to any practicable limit, the other 

 parts being increased in like proportion. It is roughly 

 estimated by the number of sparks of a given length 

 and energy which can be obtained in a given time, 

 when an uninsulated conductor is brought near the 

 prime conductor; or by the length of time required to 



