432 RECENT PROGRESS IN PHYSICS. 



the resistance to be overcome was greater, as is shown by the follow- 

 ing data. 



The ends of the wire at the break were furnished with small knobs, 

 (5.7 and 4.4 lines in diametej ;) for s = 5, g = 8, and the separation 

 of the knobs, 0.2 line, the result was as follows : 



The results were similar when metallic disks or points instead of 

 knobs were used at the place of interruption. 



Hence, the electrical discharge produces a temperature in the closing 

 •circuit as much less, as the resistance is greater, which has to be 

 overcome before discharge can take place. 



This is not a resistance which, as in the case of the interposition of 

 a long conductor in the circuit, retards the discharge throughout its 

 whole duration, but a resistance which renders discharge absolutely 

 impossible so long as it exists. 



The decrease of the heating power is always too great to be ascribed 

 to the inconsiderable residue ; hence we must draw the conclusion 

 from the above experiments that an obstacle interposed at any place 

 in the circuit being pierced by the discharge prolongs the duration 

 of the discharge through all the rest of the circuit. 



If the smallest possible charge be used for perforating mica, the 

 hole is rarely made immediately at the spot where the connexion is 

 interrupted ; the electricity almost always passes along the plate of 

 mica and penetrates at a place which, apparently, is less solid, in con- 

 sequence of a crack. If the point of application of the conductors is 

 not too far from the edge of the mica, the discharge takes place over 

 the edge. The temperature in the thermometer is as much lower as 

 the path the electricity has to traverse over the surface of the mica is 

 greater. 



The marks which the electricity leaves on the mica are very regnlar 

 and delicate, lliess has examined these as well as the corresponding 

 ones on glass. 



§ 41. Marks left by electricity upon glass and mica. — Riess placed 

 a glass plate, 0.37 of a line thick, carefully cleaned and warmed, 

 (so that when tested by the electrometer it proved itself in all direc- 

 tions a perfect insulator,) between the points of the closing wire, from 

 which the thermometer had been removed. The quantity of electrici- 

 ty, 15, collected in four jars discharged itself over the edge of the 

 plate, which was 15^ lines distant from the place where the points 

 were placed, and left marks on both surfaces, from the points of con- 

 tact to the edge. 



The marks were faint and of one color ; they grated when rubbed 



