132 Pi'of. J. C. Poggendorfi' on the Induction Apparatus 



distance of a few lines, a stream of unbroken weak sparks de- 

 scends. If the point is made to approach still nearer, until it is 

 about a quarter of a line distant, the sparks will not only be 

 clearer, but will disappear in all directions upon the glass, thereby 

 forming a vein-like figure, similar to that of Lichtenberg^s, with 

 positive electricity. No difference could be observed in the form 

 of this ligure when the point was either positive or negative, 

 except, perhaps, that with a positive point the ligure appeared a 

 little larger. Of course it is more perfect and beautiful when 

 the copper plate is covered with one thin glass plate instead of 

 with five. It may also be remarked, that even when both poles 

 of the induction coil are pointed, the action is not altogether 

 interrupted by the intervention of a glass plate. On the con- 

 trary, the points appear luminous in the dark, and emit phos- 

 phorescent sparks. In air rarefied to about 6 or 8 inches of 

 mercury pressure, the vein-like figure appears well developed 

 on both sides of the plate, the rays being more than an inch long. 

 By further rarefaction the usual phtenomenon occurs, the posi- 

 tive flame curves itself around the glass and forms a brilliant 

 luminous arch. 



It is interesting and instructive to examine these phsenomena 

 in their reaction on the induction current. 



When two glass plates, about a square foot in magnitude, 

 are each coated on one side with tinfoil and placed with their 

 uncoated sides in contact, and the coatings afterwards put iu 

 connexion with the poles of the induction coil, a continuous 

 crackling noise is heard so long as the apparatus is in activity, 

 and in tlie dark the whole intermediate space between the plates, 

 as far as the coatings extend, is seen to be filled with innu- 

 merable microscopic sparks. A piece of iodide of potassium 

 paper placed between the plates is coloured brown, on each side 

 equally. If the system is withdrawn from the apparatus during 

 its activity, and then tested, by means of the wire connecting one 

 coating, for a charge, a very weak one will sometimes be detected, 

 though oftener none whatever. This is equally the case whether 

 the poles have actually touched the coatings, or whether they 

 have been separated from them by a small stratum of air, so that 

 sparks might pass from them. It is exactly the same with a 

 Leyden jar. In both cases it will either receive no charge at all 

 or only a vei-y small one, however long it may have been exposed 

 to the current. Even when sparks are taken from one pole upon 

 the knob of the jar, whilst the outer coating and the other pole 

 are connected with the earth, it scarcely receives any charge at all. 



If, however, whilst the apparatus is in activity, and its poles 

 are either connected with the coatings of the jar or send sparks 

 to them, a wire be connected with one coating and held suffi- 



