MUSCLE 533 



Potential Current Strength Resistance. We do not know 

 what in reality electricity is, but we do know that when a current 

 flows along 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 incompressible 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 ulti- 

 mately 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 practically the case in the Daniell cell. Zinc is 

 constantly being dissolved, and the chemical energy which thus dis- 

 appears 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 alone on the difference of level between it and the 

 reservoir. 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 Darnell'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 intensity of the current will vary with 

 the electromotive force. The relation between electromotive force, 

 strength of current, and resistance were experimentally determined by 



T? 



Ohm, and the formula C = , which expresses it, is called Ohm's Law. 

 .K 



It states that the current varies directly as the electromotive force, 

 and inversely as the resistance. 



Although we do not know in what electrical resistance consists, it 

 may be defined as that property of a conductor in virtue of which a 

 flow of electricity cannot be kept up through it without the expendi- 

 ture of energy. In treating of the circulation of the blood, we have 

 already seen that the flow of a liquid along a tube involves the 

 expenditure of energy to overcome the friction of the liquid molecules 



