22 STATIC ELECTRICITY 



with an insulating handle (Fig. 19). The knobs being separated 

 a proper distance, one is first brought against the outer coating, and 

 the other is then brought near the knob of the jar ; the discharge 

 takes place by means of a spark more or less brilliant, according 

 to the charge in the jar. If the tongs are brought first against 

 the knob, discharge sometimes occurs through the operator if there 

 is faulty insulation in the handle. It is therefore advisable to 

 observe the order above described. With a given machine as 

 source of electrification the maximum charge collecting in a jar, 

 which we may take as indicating its electrical capacity, is pro- 

 portional, as we shall prove later, to the area of the coating, and 

 inversely as the thickness. It depends also on the nature of the 

 insulating material of which the jar is made. It would, for 

 example, be greater for an ebonite jar than for a paraffin jar of the 

 same dimensions. Sometimes, instead of a jar, a fiat 

 pane of glass is used, coated on it> two >i<le>. \Ve 

 may evidently regard thi" a a jar opened out. 



'Franklin's jar. If the coating is not ab>o- 

 lutely in contact with thegla^s the charge' resides on 

 the surface of the glass, passing, no doubt, through 

 the intervening thin layer of air. Thi> \\a^ pm\l 

 by Franklin by means of a jar with movable coat- 

 ings. Fig. 20 represent > the separated part" of 

 such a jar. The lowest i" a tin cup to form the 

 outer coating. The middle is a glass beaker which 

 fits into the cup, and the upper is a tin cylinder to 

 form the inner coating when dropped into the 

 beaker. After charging the- jar we mav remove 

 first the outer coating, and then the inner by an 

 insulating hook so as to prevent earth connection 

 before the coating i> removed from the jar. The 

 two coatings will now he found free from cli 

 After building up the jar again, however, a dis- 

 charge may be obtained on connecting the coating", 

 showing that \\hen the coating" wen- removed the 

 two surfaces of the glass still retained the ch 

 This may seem at liist sight inconsistent with the 

 behaviour of the electrophorus, in which the charge 

 on the cover does not pass acro the intervening 

 air to the surface of the ebonite. But it must be 

 remembered that in the jar the charge is in general 

 far greater than in the elect rophorus, and its 

 tendency to get over the intervening air space is thuefore 

 greater. 



Residual charge. If a heyden jar is charged, left for 

 a short time, and then discharged, it appear." to be entirelv fret- 

 from electrification. Hut if left again for a short time another 

 charge of the same kind as the previous one i.s found to have 



FIG. 20. 



