ELECTRICAL MEASUREMENTS. 275 



contain any electricity, it is not similar to a perfectly neutral 

 space. In fact the condition of the jar can only be expressed 

 by means of the term potential. If the jar is charged with 

 positive electricity the potential of the inside is higher than 

 the potential of the space outside. One result of this is that 

 positive electricity tends to pass away from any conductor 

 put inside the jar, and will pass away if there is a communi- 

 cation to allow it to do so. Electricity always tends to pass 

 from points of high potential, to points of lower potential 

 just as heat tends to pass from points of high temperature to 

 points of lower temperature. 



The precise definition of potential at any point may be 

 stated in this way : Suppose we have a disc charged with a 

 unit of positive electricity, and we bring it to the point in 

 question from a place where no electric force acts, that is from 

 a great distance from any charged conductor. To make 

 sure that we begin far enough away, we start at an infinite 

 distance, and carry the unit of electricity up to the given 

 position. The total amount of work that we have done 

 in bringing the electricity to that position is the potential of 

 the place that we bring it to. If v/e bring it into a conductor 

 it is the potential of the conductor. The electrical potential 

 of any point is the total amount of work we must do to bring 

 to that point a unit of electricity from a position so far off 

 that no electric force acts at the point we bring it from. 

 Instruments for measuring potential and differences of potential 

 are called Electrometers, to which class of instruments two 

 subsequent lectures are to be devoted, so that I will not enter 

 further upon this question. 



The next magnitude is the Capacity of conductors. Elec- 

 trical capacity is exactly comparable to the capacity for heat 

 of any body, or to capacity in its ordinary sense. The 

 capacity of a vessel for liquid is the quantity of liquid which 

 it can contain, only in the case of electricity the quantity 

 which a conductor can hold depends upon the means that 

 we employ for putting electricity into it. The capacity of a 

 conductor is perhaps better compared, not to the capacity of a 

 vessel for liquid, but to its capacity for a gas. The quantity 

 of air we can put into any space depends upon the pressure 

 of the air upon it. Without increasing the space we can put 

 twice as much air in if we double the pressure. So in the 

 case of a particular conductor, we can put twice as much 



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