428 Dr. A. Paalzow on the Heat of the Electric Spark. 



was replaced by two electrodes and sparks were allowed to leap 

 across the intermediate space, Knochenhauer observed a depres- 

 sion of the column of liquid, dependent on the quantity and ten- 

 sion of the electricity; but he ascribed this phenomenon not to 

 the heat generated by the sparks, but to the mechanical move- 

 ment of the air. 



The main object of the subjoined experiments was to establish 

 a connexion between the heat of the spark, the quantity and 

 tension of the electricity, and the resistance of the apparatus ; 

 and as these three magnitudes can be most readily varied and 

 increased in the case of theLeyden jar, it seemed expedient, first 

 of all, to study the thermal relations of sparks from the Leyden 

 battery. 



The heat of the spark was determined by three different 

 methods : — 



(a) By means of a thermopile and galvanometer. For this 

 purpose one face of the pile was provided with a cap of vulca- 

 nite fitting nearly air-tight. Into this cover brass electrodes 

 were screwed (air-tight) from 4 to 5 millims. apart. They were 

 so situated with regard to the pile, that sparks could not leap 

 from them on to its face. When a discharge took place between 

 the electrodes, the warmed metal and the particles of air affected 

 the pile both by radiation and conduction. 



(b) By the use of Riess's air-thermometer. 



(c) By means of a finely-graduated mercurial thermometer. 

 The following results were obtained : — 



The heat of the spark increases with the quantity and tension 

 of the electricity. With the necessary amount of resistance (by 

 which is understood that of the short thick copper wires con- 

 necting the electrodes with the outer and inner coats of the bat- 

 tery) it has its greatest value. If the resistance be increased, 

 the heat decreases and reaches a minimum. Let the resistance 

 be made still greater, and the heat again increases, attaining a 

 second but smaller maximum. By further augmenting the re- 

 sistance, it again declines till it becomes nil, in which case the 

 resistance is so great that the battery can no longer discharge 

 itself, — a condition attained by the introduction of a column of 

 water of requisite length into the circuit. 



Those degrees of resistance which lie near the point at which 

 the heat of the sparks attains its second maximum are charac- 

 terized by the following phenomena : — 



(1) The striation of light in rarefied gases. 



(2) The incandescence of the negative wire when fine wires 

 are employed as electrodes. 



(3) The absence of Abria's lines in fine powders, — a proof 

 that the air can scarcely be set in motion mechanically. 



