130 Prof. J. C. PoggendorflF on the Induction Apparatus 



between the poles of the induction wire, apparently just as strong 

 as when the current-breaker works in air. This combination is 

 further interesting, because it furnishes an opportunity of more 

 minutely observing the discharge sparks. They are here parti- 

 cularly strong and brilliant ; they spurt and hiss, particularly in 

 alcohol and oil of turpentine, and they pulverize the platinum to 

 a black powder, so that in a few moments the liquid has just the 

 appearance of ink. The last also takes place without a condenser, 

 but much more slowly and less intensely. 



Third case. — Interruption of the Induction Current by Insulators. 



When a glass plate is placed across the stream of sparks pass- 

 ing between the poles of the apparatus, it almost entirely causes 

 their cessation, provided that the poles consist of wires more or 

 less pointed at their extremities. It is otherwise when the poles 

 consist of plates. For example, if a square copper plate con- 

 nected with one of the poles be placed upon the table and covered 

 with a glass plate ; and if upon the latter a second, smaller, round 

 copper plate, connected with the other pole, be placed, a durable, 

 loud, crackling noise will be heard as soon as the apparatus is 

 set in activity; and if the experiment be made in the dark, the 

 small round disc will appear surrounded by a halo of electric 

 light, consisting of an innumerable number of small sparks in 

 unceasing motion. The smaller the round plate, the broader 

 the halo. On the lower great plate, which in this case was 3 

 inches square, no illumination was visible, although the glass plate 

 projected more than half an inch over it on each side. It is of 

 no importance with which pole the one or the other plate may 

 be connected ; it is always the smaller of the two around whose 

 edges the halo is visible. If the latter be moved about over the 

 surface of the glass plate, so that only a part of it remains ver- 

 tically above the lower metal plate, it is only this part which is 

 surrounded by sparks. If both metal plates are of the same 

 size, and the upper one is placed exactly above the lower one, no 

 halo is visible. When viewed from one side, however, it is evi- 

 dent, from the illuminated parts where the metal is accidentally 

 not in perfect contact with the glass, that from both plates in- 

 numerable small sparks pass vertically to the glass. 



Instead of one glass plate, a number of such can be placed 

 between the metals. I took five, whose total thickness amounted 

 to half an inch, and a rim of sparks was always visible around 

 the upper circular plate. This rim was certainly not so brilliant 

 as with a single glass plate ; bvit, on the other hand, with five 

 plates the limit was not reached at which the halo completely 

 disappeared. To increase the number of glass plates, however, 

 seemed the more unnecessary, as the distance of half an inch 



