6i6 A MANUAL OF PHYSIOLOGY 



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 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 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 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 intensity 

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

 between electromotive force, strength of current, and resistance 



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



XV 



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

 varies directly as the electromotive force, and inversely as the 

 resistance. 



For the measurement of electrical quantities a system of units is 

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

 the ampere, 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 con- 

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



