120 ELEMENTS OF STATIC ELECTRICITY. 



when it is in a high state of activity, there frequently 

 occurs a reversal of potential after a discharge, as shown 

 by the reversal of the brushes of light from the combs. 

 To account for this it must be considered, that the 

 residual which remains after the primary discharge may, 

 from unequal resistance, be greater on one side than on 

 the other; and after being relieved from strain by the 

 primary discharge, it will operate to give a slight pre- 

 ponderance of potential to that side, which is rapidly 

 multiplied by induction, as the rotation of the plate 

 continues. 



A reversal can also be produced by a temporary 

 reversal of rotation, as explained on page 140 ; or by 

 touching the inductors, or parts connected with them, 

 while in action, which would reduce the potential at that 

 point. Special conditions may also exist in certain 

 machines, which will reverse the ordinary mode of 

 action ; as, for instance, a difference of thickness on 

 opposite parts of a glass plate ; or in opposite jars. 



It should be noticed that the electric charge is 

 instantly diffused over the metal carriers and inductors, 

 more slowly over the paper inductors, and still more 

 slowly over the shellacked surfaces of the glass plates. 

 So that when the machine is put in action, after a con- 

 siderable interval of rest, three or four seconds elapse 

 before it becomes fully charged, and a crackling sound 

 is heard from the electricity forcing itself over the 

 resisting surfaces of the paper and glass. 



The condition of the air, as to its insulation, influ- 

 ences the whole operation of this machine. An air 

 space insulates the plates, and also the jars, with their 

 rods and balls, from each other ; and as a damp atmos- 

 phere lessens this insulation, it will decrease the energy 



