CHAPTER VII 



POTENTIAL AND CAPACITY IN CERTAIN 



ELECTRIFIED SYSTEMS. SOME METHODS OF 



MEASURING POTENTIAL AND CAPACITY 



Definition of capacity Sphere in the middle of large room Two con- 

 centric spheres Two parallel plane conducting plates Two long co-axial 

 cylinders Two long thin equal and parallel cylinders Long tl 

 cylinder parallel to a conducting plane Instrument* to measure poten- 

 tial difference Quadrant electrometer Attracted disc electrometer 

 Practical methods of measuring potential Reduction of the potential 

 of a conductor to that of a given point in the air Simple methods of 

 measuring capacity Capacity of a Leyden jar Capacity of a gold-leaf 

 electroscope. 



WE have shown in Chapter III that if we define tin- potential at a 

 point as the sum of each element of electrification divided by its 



distance from the point, or 2*, it is the work done in bringing 



unit charge from a zero-level at a distance so great that the system 

 has no appreciable action there. We may frequent!? bring the 

 zero-level quite close to the system. Thus, if a room has conduct- 

 ing walls, an electrified system within it has no intcn>itv outside, 

 and the walls may be regarded as having zcro-le\el. We shall 

 now consider some special systems which will illustrate the use of 

 the formula for the potential. 



Infinitely thin small metal plate held normally to the 

 lines of force or strain. The plate is to be so small that the 

 lines of force may be regarded as straight and parallel in the region 

 just about the point at which it is introduced before that introduc- 

 tion takes place. Then, when it is introduced, since it is perpen- 

 dicular to the previous course of the lines, the positive charge on 

 one side will evidently be equal, element by element, to the negath e 

 charge on the other side. The potential due to the plate at any 

 point outside it will be made up of equal and opposite terms, since 

 for every element of positive charge there is an equal element 

 of negative charge practically at the same distance from the point, 

 as the plate is infinitely thin. Then the plate does not affect the 

 potential in its neighbourhood, and therefore does not alter 



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