■ 16 Mr. Faraday uml Dr. P. lliess on the Action of 



The shell-lac disc was moved only once from above to below be- 

 tween the positively electrified conductor of a machine and a 

 spirit-flame; the anterior face of it was laid with sliding contact* 

 on the knob of a gold-leaf electroscope ; the disc was with- 

 drawn and the electricity in the electroscope examined. Always 

 negative electricity was found, weaker or stronger; the strongest, 

 when the centre of the large disc had touched the knob, and it 

 was carefully breathed upon, whereof the reason is obvious. I 

 have imputed to the flame the essential part of destroying the 

 positive electricity of the posterior face. You have observed the 

 fact that the posterior face is negatively excited, and you have 

 hence drawn some consequences concerning the mode of induc- 

 tion on the plate which I cannot admit. The fact of the pos- 

 terior face being negatively electrified appears to me a very 

 complicated one, and resulting from one of the two following 

 causes, perhaps from both : — First, the flame is inducteously 

 excited by the originally electrified body and imparts its negative 

 electricity to the posterior surface; secondly, the negatively 

 electrified anterior face of the disc acts by induction upon the 

 posterior face. Concerning the first assumption, I have con- 

 cluded from experiments made on the electric properties of 

 burning bodies (Poggendorff's Annalen, vol. Ixi. p. 545), that a 

 flame electrified by induction acts upon a body in its vicinity by 

 means of its electricity, which is contraiy to that of theinductric 

 body. As to the second assumption, a decisive experiment, as it 

 appears to me, has been made and described by me in vol. i. § 300 

 of my work on Electricity. A shell-lac disc was held, by means 

 of a handle, freely in the air, and rubbed upon one surface (we 

 will say, the superior) strongly with fur. Although it cannot 



* I gave the one motion between the inductric body and the flame, and 

 obtained precisely. the same results as those described in my letter. It is 

 qnite easy to ascertain which surface of the plate np is charged, and whether 

 positive or negative, without ever making contact with the ball or cap-plate 

 of the electrometer, by a near approach only. I believe it to be essen- 

 tially necessary to avoid a sliding contact between the shell-lac plate and 

 the metal ball of the electrometer, for I find that by employing a perfectly 

 uncharged plate and instrument and making such a contact, electricity is 

 excited, the shell-lac becomes positive, and the metal negative, so that the 

 moment the shell-lac is withdrawn the electrometer diverges with negative 

 electricity. When a charged piece of diy shell-lac, made positive by fiiction 

 with metal and to a degree enough by near approach to diverge the gold- 

 leaves of an electrometer an inch or more, is employed, I find it impossible 

 to convey that charge to the dry instrument by friction against its metal 

 cap ; the shell-lac only becomes more positive, and leaves the instrument 

 in the negative state : therefore I doubt the simple commimication of ne- 

 gative electricity from weakly-charged, dry, insulating shell-lac to dry metal 

 by friction contact ; though I expect in every case excitement and evolution 

 of electricity, and that the electrometer will be rendered negative and the 

 shell-lac positive. — M. F. 



