140 ELEMENTS OF STATIC ELECTRTCITY. 



side coatings of this jar ; and, in case of fracture, which 

 sometimes occurs from an overcharge, it is always this 

 jar which is broken : and the fracture always occurs on 

 the side nearest the opposite jar, showing that the 

 charge is attracted to that side, and electricity repelled 

 from the outside coating, creating a sufficient difference 

 of potential between the two coatings to overcome the 

 resistance of the glass and perforate it. 



REVEKSAL OF POTENTIAL. It has been already 

 stated that there is frequently a temporary reversal of 

 potential. Such a reversal can be produced, if desired, 

 by joining the electrodes P and 72, and reversing the 

 rotation of the plate till the machine is fully discharged; 

 then separating P and R while the reversed rotation is 

 continued, and then resuming the direct rotation, when 

 a complete reversal of potential will be found to have 

 occurred, which will continue till again reversed by a 

 similar experiment, or till the machine has had a period 

 of rest. The explanation is as follows: 



When P and R are separated, and the rotation re- 

 versed, the same causes which before operated to raise 

 the potential of D above that of 6 7 , now operate to raise 

 the potential of O above that of 7), but in a very lim- 

 ited degree. For, as already shown, any carrier, as W, 

 charged by the brush E, would now give up its princi- 

 pal charge to the brush and comb at V; but the resid- 

 ual, slightly increased by the inductor X, would be 

 given up, through the comb TTto the jar O; while the 

 opposite carrier Z, would give up its principal charge 

 at H, and carry its residual to the comb L, and the jar 

 Z>, after a slight increase by the inductor T. But the 

 difference of insulation between the upper and lower 

 parts affect these residual discharges in the same man- 



