VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 'iti" 



gree. But if those imperfectly conducting substances were dried too much, 

 then they would become quite electrics, and consequently useless for our pur- 

 pose, excepting when they were used like resins, &c. 



The apparatus may be rendered more simple by applying the silk, or other 

 semi-conducting stratum, to the upper, viz. to the metal plate, which is fur- 

 nished with a glass handle, instead of the marble or other plate, which in that 

 case becomes useless : for in its stead a plane of any kind may be used, such as 

 a common wooden or marble table, even not very dry, a piece of metal, a book, 

 or other conductor, whether perfect or imperfect, it being only necessary that its 

 surface be flat. In fact, nothing more is requisite for the experiment, than that 

 the electricity, which tends to pass from one surface to the other, should find 

 some resistance or opposition in either of the surfaces. It is immaterial whether 

 the non-conducting or semi-conducting stratum be laid on one or the other of 

 the planes, it being only necessary that they should coincide very well together, 

 which cannot be easily obtained when a common table is used for one of the 

 planes, which is the only reason why it is better to use two planes which have 

 been worked flat by grinding one upon the other, and one of them varnished, 

 &c. A single metal plate, covered with silk, with 3 silk strings fastened to it by 

 way of a handle, may be conveniently used for ordinary experiments. 



Hitherto has been considered the use of the condenser in exploring the weak 

 atmospherical electricity, which is brought down by the atmospherical conductor.* 

 But this, though the principal, is not the only use to which it may be applied. 

 It serves likewise to discover the artificial electricity when this is so weak as not 

 to be discoverable by any other means, which happens in various cases, as for 

 instance : a Leyden phial charged, and then discharged by touching its coated 

 sides 3 or 4 times with the discharging rod, or the hand, seems to be quite de- 

 prived of electricity ; yet if you touch with the knob of it the metal plate of our 

 condenser, when properly situated (viz. on an imperfectly conducting plane, &c.) 

 and immediately after take up the said plate, this will be found to give very con- 



* Here it will be proper to mention a remarkable observation which I have made on the atmos- 

 pherical electricity with the help of the condenser. The late Mr. Canton and others affirmed that 

 they had obtained stronger signs of electricity from their atmospherical apparatus at the time of an 

 aurora borealis, than at other times ; but various other philosophers doubted of the influence of 

 electricity in that meteor, and some absolutely denied it. I myself was much in doubt about it j 

 but at present Mr. Canton's assertion seems to be established beyond a doubt, as I have observed by 

 actual experiment. During the strong aurora borealis, which appeared in the night of the 'JSth 

 of July, 1780, the light of which rising gradually from the horizon, reached the zenith at near 1 1 

 o'clock, and enlightened the heavens with a reddish light, the weather being clear and windy ; our 

 condensing apparatus being applied to an atmospherical conductor, gave fine bright sparks ; whereas, 

 at other times, that is, in clear weather, and at every hour of the day or night, the same apparatus 

 afforded either no sparks at all, or exceedingly small ones ; die reason of which is, because the said 

 . onductor was not much elevated. — Orig. 



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