620 THE LEYDEN JAR. 



metallic coatings by a narrow strip of tinfoil which is pasted on 

 the glass. To charge the plate the insulated metallic side is con- 

 nected with the machine, and if the other side communicates with 

 the ground, the two coatings act exactly like the two discs in the 

 apparatus previously used. If connection between the two coatings 

 be made by touching them with the hands a violent shock is felt. 



V The Leydtn jar (fig. 323) is a condenser which differs 

 from Franklin's plate only in its shape, the coatings and 

 the insulating plate being not flat but rolled up in the 

 form of a jar. A metal rod, which ends outside in a 

 knob and communicates within the jar with the interior 

 coating of tinfoil, serves for charging the interior with 

 electricity. 



If the student does not possess an electrical machine, and is con- 

 fined to the glass rod and the electrophorus as his sources of elec- 

 tricity, two Leyden jars are quite sufficient, 

 one 5 or 6 cm wide and from 7 to 9 cm high, 

 the other 8 or 10 cm wide and 12 or 16 cm 

 high. For use with the machine, one or 

 several (from two to four) larger jars are 

 desirable, about 15 or 20 cm wide and from 

 24 to 32 cm high. 



Bottles of hard glass with very wide necks, 

 such as common pickle bottles, are the most 

 convenient for attaching the inner coating. 

 Such bottles of white glass may be pur- 

 chased at the dealer's ; the common pickle 

 bottles are mostly of green glass, which does 

 (an. proj. real size)\ no ^ look so well, but serves the purpose quite 



as satisfactorily. 



Before coating the jar its insulating quality should be tested. 

 The jar is rubbed with a dry cloth, especially the upper part, and a 

 piece of silvered paper, broad enough to cover about two-thirds of 

 the height of the jar and so long as to pass amply round it, is 

 wrapped firmly round the jar, leaving an edge beyond the bottom 

 about l cm wide. The paper is tied to the jar with thread, and the 

 projecting edge is bent in so as to be everywhere close to the 

 bottom of the vessel. Iron filings are then thrown into the vessel 

 until it is about two-thirds filled with them. The vessel now re- 

 presents a temporary Leyden jar with two coatings, and a metal 



