MECHANICAL EFFECTS OF THE DISCHARGE. 639 



If the jar is charged by an electrophorus, a compact writing 

 paper, not too stout, should be used ; good note paper will mostly 

 answer the purpose. In paper which is not close, such as filter or 

 blotting paper, the fine perforations are not well seen. With a 

 machine and a large jar, still more with a battery, very stout paper, 

 playing cards, and even several thicknesses of note paper, may be 

 perforated. The object to be perforated is placed upright upon the 

 table of the universal discharger, the opposite ends of the two wires 

 are moved until they press against the object on either side, and 

 the discharge is conducted precisely as in the experiment for fusing 

 the strip of tinfoil. f 



With a single good-sized Leyden jar of very stout glass it is even 

 possible to perforate a glass plate. Glass being a bad conductor, it 



FIG. 331 (an. proj.\ \ real size). 



presents considerable resistance to the passage of electricity through 

 its substance, and it is therefore in this experiment first of all ne- 

 cessary to assign to the discharge the shortest route possible ; this is 

 jdone by arranging the two pointed ends of the wires, between which 

 the spark has to pass, so as to be in close contact with the glass and 

 exactly opposite to each other. Next, the plate must be very thin, 

 the thinnest window glass obtainable, and, further, care must be 

 taken that the discharge does not find its way round the edge of 

 the glass, either through the air or along a conducting layer of 

 moisture which may possibly have collected upon the plate. In 

 order to prevent this, the ends of the wires may each be surrounded 

 by a little resin fixed on each side to the plate ; it is, however^ 



