PRELIMINARY DATA 725 



a wire energy is expended, just as energy is expended when water flows 

 from a higher to a lower level. Many of the phenomena of current 

 electricity can, in fact, be illustrated by the laws of flow of an incom- 

 pressible liquid. The difference of level, in virtue of which the flow 

 of liquid is maintained, corresponds to the difference of electrical level, 

 or potential, in virtue of which an electrical current is kept up. The 

 positive pole of a voltaic cell is at a higher potential than the negative. 

 When they are connected by a conductor, a flow of electricity takes 

 place, which, if the difference of level or potential were not constantly 

 restored, would soon equalize it, and the current would cease; just as 

 the flow of water from a reservoir would ultimately stop if it was not 

 replenished. If the reservoir was small, and the discharging-pipe large, 

 the flow would only last a short time ; but if water was constantly being 

 pumped up into it, the flow would go on indefinitely. This is prac- 

 tically the case in the Daniell cell. Zinc is constantly being dissolved, 

 and the chemical energy which thus disappears goes to maintain a 

 constant difference of potential between the poles. Electricity, so to 

 speak, is continually running down from the place of higher to the place 

 of lower potential, but the cistern is always kept full. 



The difference of electrical potential between two points is called 

 the electromotive force; and from its analogy with difference of pressure 

 in a liquid, it is easy to understand that the intensity or strength of the 

 current that is, the rate of flow of the electricity between two points of 

 a conductor does not depend upon the electromotive force alone, 

 any more than the rate of discharge of water from the end of a long- 

 pipe depends alonfe on the difference of level between it and the reser- 

 voir. In both cases the resistance to the flow must also be taken account 

 of. With a given difference of level, more water will pass per second 

 through a wide than through a narrow pipe, for the resistance due to 

 friction is greater in the latter. In the case of an electrical current, a 

 wire connecting the two poles of a Daniell's cell will represent the pipe. 

 A thick short wire has less resistance than a thin long wire; and for a 

 given difference of potential, of electric level, a stronger current will 

 flow along the former. But for a wire of given dimensions, the in- 

 tensity of the current will vary with the electromotive force. The 

 relation between electromotive force, strength of current, and resistance 



17 



were experimentally determined by Ohm, and the formula C= ~ 



which expresses it, is called Ohm's Law. It states that the current 

 varies directly as the electromotive force, and inversely as the resist- 

 ance. 



For the measurement of electrical quantities a system of units is 

 necessary. The common unit of resistance is the ohm, of current 

 the ampdre, of electromotive force the volt. The electromotive force 

 of a Daniell's cell is about a volt. An electromotive force of a volt," 

 acting through a resistance of an ohm, yields a current of one ampere. 

 But the current produced by a Daniell's cell, with its poles connected 

 by a wire of i ohm resistance, would be less than an ampere, because 

 the internal resistance of the cell itself that is, the resistance of the 

 liquids between the zinc and the copper must be added to the external 

 resistance in order to get the total resistance, which is the quantity 

 represented by R in Ohm's Law. 



Measurement of Resistance. To find the resistance of a conductor, 

 we compare it with known resistances, as a grocer finds the weight of a 

 packet of tea by comparing it with known weights. The Wheatstone's 

 bridge method of measuring resistance depends on the fact that if four 

 resistances, AB, AD, BC, CD, are connected, as in Fig. 231, with each 



