Prof. Dr. Jacobi 07i the Galvanic Spark, 40S 



zinc plates were carefully amalgamated, and the charge was 

 composed of 100 measures of water and 8 of sulphuric acid. 



It resulted from frequently repeated experiments that the 

 points might be approximated to within a distance of 0*00005 

 English inch of each other without a spark passing : the bat- 

 tery might be either connected into a single pair of plates, 

 or arranged as a pile of twelve pairs. Instead of the copper 

 points a small copper ball of 0-1 inch in diameter was now 

 screwed to one side, and even then no spark passed at the 

 above-mentioned distance ; the same was the case after the 

 ball had been amalgamated so strongly that a layer of 

 bright fluid mercury still adhered to it. It must therefore be 

 admitted that with a distance of the surfaces of contact of 

 0'00005" English, no evident spark yet passes, either on em- 

 ploying a simple circuit, or with a battery of 12 pairs. I 

 also inserted in the galvanic circle a very sensitive galvano- 

 meter (Nobili's) with double needle, at the distance of the 

 surfaces of contact above stated, when the needle was not in 

 the least affected. Since however the prepared frog passes 

 for a still more sensitive galvanometer, it would be desirable 

 to ascertain at what distance of the surfaces of contact this 

 would exhibit contractions. Not less desirable is it to know 

 the distance at which Professor Moser has seen the thermo- 

 magnetic spark and that of the Becquerel's circuit pass (ii. 

 p. 113.). 



If therefore, provisionally, a spark previous to contact is 

 not proved, the reason already stated falls to the ground, and 

 no other exists for concluding on a particular direction of the 

 particles of oxygen towards the zinc which takes place even 

 previous to complete contact (i. p. 190.). 



What takes place when the contact is completed and broken 

 again, is therefore simply a phaenomenon of combustion. We 

 may, however, plainly observe how the surfaces of contact 

 immediately become coloured, or under circumstances red hot. 

 It depends therefore on the proportion of the heating power 

 of the current to the magnitude of the surfaces of contact, 

 whether immediately a larger or smaller particle is ignited 

 and detached. It is always observed that when immediately 

 at contact a spark has been produced, the surfaces of contact 

 have again, after the combustion, separated from each other. 

 The colour of the spark on employing copper points was at 

 first green ; if the experiment was frequently repeated, it ap- 

 peared to me afterwards darkish red ; after more frequent re- 

 petition, the layer of oxide formed prevented the metallic con- 

 tact and no further spark appeared. This layer of oxide is pro- 



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