GENERAL ACCOUNT OF COMMON PHENOMENA 21 



combs will clearly tend to draw out and leave + on the right- 

 hand discharging rod. On the left-hand rod, will be left, and 

 if the knobs are not far apart the charges may soon be sufficient 

 to spark across. But if the knobs are so distant that no sparks 

 pass, the charges on them may soon reach a steady state, and no 

 more electrification being taken from the combs, the machine works 

 just as if the discharging circuit were removed, and increase is 

 balanced by leakage. 



Storage of electrification. The Leyden jar. The 

 Leyden jar is a device for collecting and storing large quantities of 

 electrification. It usually consists of a glass jar, as shown in Fig. 

 18, lined inside and out with tinfoil to perhaps three-quarters 

 of its height ; the neck of the jar is closed by a cork, or preferably 



FIG. 



FIG. 19. 



some insulating material, through which passes a metal rod termi- 

 nated above in a knob and below in a chain, which puts it in 

 communication with the inner coating of tinfoil. If the j.ar is 

 held in the hand, the outer coat is connected to earth, and 

 if the knob is applied to the prime conductor of a machine, the 

 positive charge from the prime conductor passes into the jar and 

 collects on the outside of the inner coating. There it induces a 

 negative charge on the inside of the outer coating. If the knob is 

 held at some distance from the prime conductor the jar is charged 

 by a scries of sparks, and the successive rushes of negative elec- 

 tricity to the outer coating through the arm may frequently be felt. 

 If the source gives large charges in each spark, the rushes through 

 the arm would be painful, and the arm must be replaced by an 

 earth-connected wire. 



To discharge the jar it is only necessary to connect the two 

 coatings, or to connect the inside to earth. As it is disagree- 

 able to the operator to form part of the connecting circuit it is 

 uMial to employ a discharger, consisting of metal tongs provided 



