ELECTRICAL LEVEL OR POTENTIAL 73 



middle of the room as being at a higher electrical level than the 

 \\alls and think of the level as gradually falling from the body to 

 the walls in every direction. The conductor is all at the same 

 level, for the charged carrier will not move along its surface, but 

 will be pulled straight out from it at every point. The walls, if 

 conducting, are also at one level, since the intensity is everywhere 

 perpendicular to the surface, and the carrier will not tend to move 

 along the surface at all. 



The gravitative analogy, if followed out, also suggests a definite 

 method of measuring electrical level. We usually measure difference 

 of level above the earth's surface in vertical feet or metres. But 

 this measurement will not give us consistent results in extended 

 n stems. For think of two canals at different levels reaching from 

 the latitude of London to the Equator. Suppose the surface of the 

 upper one is 978 cm. above that of the lower at London. At 

 the Equator the difference in level will be, not 978, but 981 cm. 

 For the work done in letting a given mass of water down from one 

 canal to the otluT must be the same at each end. Otherwise it 

 would be possible to get an endless store of energy out of the water 

 by allowing it to trickle down at the end where the most work was 

 i out. and t<> do work as it fell. We might use some of this 

 work to raise an equal mass <>f water up again at the other end and 

 we illicit transform or store the balance. This is contrary to all 

 experience, so that we are certain that the work done in lifting a 

 gramme of water from one level to the other is the same at each 

 end and indc.-d at every intermediate point. Since then g at 

 Ion : g nt the Kqimtor = 981 : 978, if one canal is 978 cm. 

 higher than the other at London, it mu>t be 981 em. higher at the 

 Equator. It is worth noting that if we were concerned with exact 

 measurements of work done in 



ing mosses, our Ordnance 



: >s .should be marked, not in 

 feet above sea- level, but in ergs 

 >t-poundals. 



Now, turning to an electrical 

 system, let us imagine a surface 

 Sj, Fig. 60, drawn in the dielec- 

 tric all at one level, drawn, that 

 is, so that at every point of the 

 surface the force on a small FIG. 60. 



charged carrier is perpendicular 



to the surface, and no work is done in moving the carrier along 

 tht 1 . Imagine another sin face S 2 in the dielectric, also 



. but at a higher level than the first. Then the work done 

 in taking a small charged carrier from the first surface to the 

 second is the same by all paths, l-'or imagine that it is greater 

 by one path. Ah, than by another path, CD, from the first 

 surface, S r to the second surface, S r Taking the carrier round 



