58 PHYSIOLOGY OF MUSCLE AND NERVE 



is 1.5 volts. The ordinary type of dry cell consists of a zinc jacket lined with 

 plaster of Paris and saturated with ammonium chlorid. Its inner space is taken 

 up by a carbon plate surrounded by black oxid of manganese. 



While the nature of electricity has not been recognized as yet, we know that ^n 

 electrical current passes over a system of wires in the same manner as water flows from 

 a high to a low level. It leaves the generator at its place of high electrical potential 

 and reenters it at its place of low potential. The point of exit forms the positive 

 pole or anode (ana = up) and the point of entrance, the negative pole, or cathode 

 (cata = down). The difference in the potential between these two points is 

 designated as the electromotive force. It is easy to understand that this difference 

 can only be kept up if there is a constant supply of current. As the zinc is being 

 dissolved, the chemical energy liberated thereby tends to maintain a constant 

 electrical pressure at the two poles. The cell, therefore, represents a reservoir of 

 electricity which remains filled as long as there is sufficient material present to 

 generate chemical energy. If, however, the material is used up, the difference in 

 potential can no longer be maintained and an equalization must finally result 

 which causes the current to cease. In this regard electricity behaves like water, 

 because the flow of the latter from a reservoir continues only as long as the outgo 

 is balanced by an adequate ingo. 



While traversing a system of wires the electrical current loses a certain amount 

 of its initial energy, owing to the resistance which it must overcome. Hence, the 

 strength of the current or the rate of flow of electricity between two different points 

 of a conductor is dependent not only upon the electromotive force but also upon 

 the resistance resident in the conducting path. Obviously, if the poles of a cell are 

 connected by means of a short and thick wire, the resistance to be overcome will be 

 less than if joined by a long and thin wire. In the former case, therefore, the flow 

 of electricity will be greater than in the latter, provided, of course, that the electro- 

 motive force remains unaltered. It must also be evident that the strength of a 

 current through a certain length and thickness of wire must be directly proportional 

 to the electromotive force. In addition to this external resistance which the elec- 

 trical current encounters in its passage through a conductor from copper to zinc, 

 it must also overcome the internal resistance, resident in the constituents of the cell 

 between the zinc and copper. Provided that the conducting power of the liquid 

 remains the same, the resistance must decrease with the size of the plates and 

 increase with the distance between them. 



Measurement of Electrical Quantities. — In accordance with the 

 metric system, a unit of current is designated as an ampere, a unit 

 of electromotive force as a volt, and a unit of resistance as an ohm. 

 An ohm equals the resistance of a column of mercury 1 mm. in cross- 

 section and 1063 mm. in length at 0° C. The electromotive force or the 

 electrical pressure, so to speak, of a Daniell cell is about one volt. 

 If this power is permitted to act through a resistance of one ohm, 

 a current of approximately one ampere is obtained. In the case of the 

 Daniell cell, however, the amperage is really somewhat smaller, because 

 even if the outside wire p>ossesses a resistance of only one ohm, the total 

 resistance to be overcome by the current is actually greater, owing to 

 the fact that it is also opposed by the internal resistance of the cell. 

 The relationship existing between these different factors has been 

 determined experimentally by G. S. Ohm (1827), in accordance with 

 the following formula : 



^ X X ^1 electrom. force volts ^. 



Current strength = ^p- t^ftt or amperes = -r — * Smce 



'^ Int. res. + Ext. res. ^ ohms 



these factors are very closely related, it is possible to determine any 



