430 RECENT PEOGRESG IN PHYSICS, 



"be brought into contact before the current can be produced. After 

 the carbon points have once been in contact, they may be separated 

 from each other and then the luminous arc is formed. The arc also 

 can be produced by discharging the spark of a Leyden jar between the 

 carbon points, instead of bringing them into contact, as has been 

 observed by Daniell, (Phil. Trans., 1839, 89 ; Pog. Ann. LX, 312,) 

 and previously by Sturgeon. — (Ann. of Electr., VIII, 507 ; Pog. Ann. 

 XLIX, 122.) The latter believes that this experiment originated 

 with Herschel. The formation of the current in this case is evidently 

 caused by conducting particles carried from one pole to the other by 

 the spark. 



Even the powerful batteries which have been u.^ed to produce the 

 luminous arc have not yet sufficient tension to effect the passage of a 

 spark through the smallest distance. 



Jacobi found by accurate measurement that the poles of a battery 

 of 12 zinc-platinum elements could be brought to within 0.00005 inch 

 of each other without a spark passing. — (Bulletin of the Petersburgh 

 Academy; Pog. Ann., XLIV, 633.) 



Gasf-iot obtained distinct sparks from his great water batterv. — 

 (Phil. Trans , f. 1844, pt. I, pp. 39 ; Pog. Ann , LXV, 476.) this 

 remarkable battery consisted of 3,520 glass tumblers, each containing 

 a zinc rod and a copper cylinder. They were charged with rain 

 water. The tumblers were distributed upon 44 oaken boards, every 

 11 of which were combined in a stand, similar to the shelves of a book 

 case, and were supported by four strong pillars. Especial care was 

 taken to secure perfect insulation, the glasses were varnished and 

 placed upon glass plates, and these as well as the boards were in like 

 manner covered with varnish. 



It might be expected that this battery, when the circuit was unclosed, 

 should exliibit in a decided manner the phenomenon of tension, and, 

 in fact, the leaves of a gold leaf electroscope diverged already, when 

 yet at a distance of 2 or 3 inches from one of the poles. 



As soon, however, as the battery was closed, all signs of tension 

 disappeared. When the poles of the battery were brought within 

 0.02 inch of each other sparks continually passed between them. lu 

 one case this phenomenon continued day and night for five weeks 

 without interruption. Several months after its constructiou the 

 battery showed no signs of decrease in strength. 



fctince the ordinary galvanic batteries have no striking distance at 

 all, it is evident that the appearance of light observed in opening and 

 closing even simple batteries, must be an entirely different phenomenon 

 from that of the common electric spark. 



In the memoir just mentioned, Jacobi states it as his opinion that 

 the usual galvanic spark is a phenomenon of ignition and combustion ; 

 the extremely fine points which first come into contact and permit the 

 conduction of the current, become incandescent and burn, and thus pro- 

 duce, according to Jacobi's opinion, the phenomenon of light. There 

 is no doubt that with more vivid sparks such ignition and combustion 

 really do occur, but then the phenomenon is no longer a simple one ; ' 

 in closing and opening a battery the spark is observed under circum- 

 stances which make ignition and combustion very improbable. 



