66 Observations on some Experiments in Electricity. 



may be prevented by rolling the card up in the form of a cylinder, 

 and placing the two poles, one within and the other without the enclo- 

 sure. It would indeed be far more extraordinary to find the perfo- 

 ration uniformly opposite to the positive pole \n vacuo, than to the 

 negative, when the experiment is made in air. 



The facts above stated, taken in connexion with those discovered 

 by M. Ermann respecting the unequal degree in which the two elecr 

 tricities are insulated by other substances besides air, (such as flame' 

 of alcohol and phosphorous, dry alkaline soap, and liquid sulphuric 

 ether,) furnish a strong proof that the experiment of Lullin cannot 

 be claimed as peculiarly favorable to the uni-fiuid theory. 



8. The form and arrangement of macJdnes. 



The advantage of different constructions for electrical machines 

 was formerly discussed, by philosophers, and attempts have been 

 made to explain the manner in which, the plate and cylinder machines 

 respectively operate, to produce results so difl'erent from each other. 

 Some of these explanations, although hardly deserving the name, ap- 

 pear to be generally acquiesced in. It has been remarked that " the* 

 plate machine furnishes a more abundant quantity of the electric fluid 

 than that with the cylinder, and that if the two machines furnish 

 sparks of the same length, the spark from the conductor of the plate 

 machine is much more active and more pungent than the spark from the 

 cylinder machine." This difference is accounted for by saying that "the 

 plate is rubbed on both sides, and the electricity taken away by the col- 

 lector from one only of those surfaces ; whereas in the other kind of 

 machines, the outer surface only of the cylinder is rubbed, and the 

 electricity is received immediately by the collector." But to say 

 nothing of the experiments of Mr. Nicholson which proved that rub- 

 bing both surfaces of the plate gained no more electricity than to con- 

 fine the operation to one; we may, I think, satisfactorily account for 

 the difference by considering that the diameter oiihe plate is usually 

 much greater than that of the cylinder machine, whence the electrici- 

 ty developed on the glass, is carried by the former, much farther out 

 of the influence of the rubber, than by the latter. 



The intensity of the spark is accordingly increased in the same 

 manner as that of an electrophorus plate is augmented by removing 

 it farther and farther from the resinous electric on which it usually 

 rests ; and when the negative and positive points of the machine are 



* See Rces's Cyclopaedia, article " Electrical.'''' 



