RECENT PROGRESS IN PHYSICS. 431 



Bless has clecarly explained this in the following ingenious manner: 



The separation of the plates involves two conditions which act in 

 opposite ways upon the temperature of the wire in the thermometer. 

 One is the part of the electricity remaining in the battery — in con- 

 sequence of which evidently the heating must be diminished. The 

 other condition which, on the contrary, raises tlie temperature, 

 requires a more extended explanation. 



When the distance between the two discs is less than the striking 

 distance of the battery, one spark passes between tlie knob of the 

 battery and the knob of the discharger, and a second between the 

 plates. 



When the distance between the discs is greater than tlie striking 

 distance, a spark can pass between the discs if tlie knob of the dis- 

 charger is in contact with that of the battery. The passage of the 

 spark between the plates is only possible because, generally, as we 

 have seen above, (page 423) the striking distance between plates 

 is greater than between knobs. 



At the passage of the spark between discs, a condensation of elec- 

 tricity takes place at their edges, and tliis condensation, very probably, 

 has an accelerating effect upon the discharge which shows itself by an 

 increase of temperature. 



This last condition, Avhich raises the temperature of the closing wire, 

 can appear only when the distance between the plates is greater than 

 the striking distance between the knobs. Let the plates stand at a 

 given distance. The striking distance between the knobs changes 



with the power of the charge ; it is proportional to the fraction — . 



s ,' 

 for weak charges it is small, for stronger charges it increases ; hence, 

 it is in weak charges only that the above mentioned acceleration of 

 the discharge can increase the temperature so much that the opposite 

 influence of the residual charge shall be overpowered. 



In fact, we see in the above table that h, when the plates are 1 line 

 apart, only when s = 3 and <? = 3, s z= 4 and <? = 4, s =i 4 and <? == 5, 

 is greater than h in the case of contact of the plates. In all these cases 



-- is not greater than 1.25. For charges so powerful that ~ is greater 



than 1.25^ the temperatures of the last column, as a whole, are less 

 than the corresponding temperature in the case of contact of the plates. 



If the separation of the plates is greater than tlie possible striking 

 distance, of course there is no discharge. 



The results were similar when small balls were used instead of plates, 

 the striking distance between the small balls being a little greater 

 than that between the large ones of the discliarger and the battery ; 

 hence, in a favorable case, the temperature, at a distance of the small 

 knobs of only 1 line, was very little higher than when they were in 

 contact. 



§ 40. Heating power of obstructed discharge. — When a thin in- 

 sulator was introduced at the place of interruption, through which 

 the discharge stroke could penetrate, the heating power was less, as 



