LABORATORY MANUAL OF PHYSIOLOGY. 



such as zinc and copper, partly immersed in a dilute acid or solu- 

 tion of a salt. It is now considered that the current is not due so 

 much to the difference of potential of the metals as it is to the dis- 

 sociation of the solution into its so-called ions. These are sup- 

 posed to be charged with positive and negative electricity'. Thus, 

 if H a SO t be used as the electrolyte, the H group represents ions 

 which are electropositive, while the SO 4 group is electronegative. 

 These ions, charged with positive electricity, move toward the nega- 

 tive pole, in this case, zinc, while those charged with negative elec- 

 tricity, move toward the positive pole, in this case, copper. The 

 former are termed kations; the latter are known as anions. When 

 the tw r o metals are connected by a conductor, there is a flow of 

 current from the place of highest intensity, the anode, to that of 

 lower intensity, the cathode. This may be compared to the flow 

 of water from areas of high pressure to areas of lower pressure. 

 The energy upon which the flow depends is known as. the electro- 

 motive force (E.M.F.). Its unit of measurement is the VOLT. 



The flow of the current meets with more or less resistance which 

 has to be overcome. This resistance is inversely proportional to the 

 length and thickness of the conducting substance. It also varies 

 with the nature of the substance itself, irrespective of its length and 

 thickness. There is resistance in the cell itself. This is known as 

 "internal" resistance to distinguish it from that in the wire which 

 is known as external resistance. 



Where the external resistance is high, as in passage through tis- 

 sues, the internal resistance of the cell is a negligible quantity. 



2. Ohm's Law. The relations between electromotive force, re- 

 sistance, and intensity of current, are formulated as Ohm's law. 

 If I represents intensity of current, E.M.F., electromotive force, 



R 



and R, resistance, then I = - 



E.M.F. 



The unit of measurement for the strength of current is the Am- 

 pere. This represents the quantity of electricity passing over a 

 given cross-section of conductor in a given time. That amount of 

 electricity which will deposit 0.001118 grams of silver per second 



[14] 



