Dr. A. Paalzow on the Heat of the Electric Spark. 429 



It appears therefore that intense heating of the sparks by 

 great resistance in the circuit, striation of light, and incandes- 

 cence of the negative wire are only observed when the duration 

 of discharge has been augmented by great resistance, and when 

 the velocity of electricity, and consequently the mechanical move- 

 ment of the medium in which the spark appears is very small — in 

 which case only could we first predicate of the air that it plays 

 the part of conductor. 



Were it desired to establish a connexion between these results 

 of the dependence of the heat of the spark on resistance, and 

 the known laws of the rise in temperature of metals and liquids 

 by means of electric currents, we should have to assume, in the 

 case of the sparks between the first maximum and first minimum, 

 that the incandescent metallic particles w T hich are projected into 

 the air along the course of the spark conduct almost exclusively 

 and become as much heated as if they constituted, as regards re- 

 sistance, a measurable part of the entire circuit. It follows from 

 this, in the case of the sparks near the second maximum, on the 

 other hand, that the air is almost exclusively the conductor, and 

 its resistance is very great in comparison with the whole of the 

 remainder of the circuit, no matter whether this consist of solid 

 or liquid bodies. 



The sparks near the second maximum presenting a great simi- 

 larity to the electric brush, those of the electrical machine were 

 also examined in respect to their heating-powers. It was found 

 that " brush " sparks with hissing noise develope much more 

 heat than those of any other kind. The form of the electrode has 

 consequently an influence on the heat of the spark, the brush 

 being formed with less or more difficulty according as the elec- 

 trodes are pointed or rounded. 



The results which this inquiry into the heat of the electric 

 spark has furnished are therefore well worth consideration in the 

 construction of electrical exploding-apparatus. For it has been 

 shown by experiments on another occasion that fuses divide them- 

 selves into two classes — those which can be fired by any kind of 

 electric spark, and those the ignition of which only takes place 

 by sparks near the second maximum. If therefore a fuse of the 

 latter class be chosen, such a degree of resistance must be 

 given to its exploding-apparatus that sparks of this kind are 

 produced. 



