578 PHYSIOLOGY. 



ling cells abreast or in multiple arc, which amounts to the same thing 

 as having a cell with large-sized zincs. 



To summarize: to obtain increased intensity of current with 

 small external resistance, as in a cautery wire, either use large cells 

 or couple a number of cells abreast or in multiple arc; with great 

 external resistance, as in the application of the galvanic current to the 

 human body or the nerves of an animal, you couple the cells in series, 

 small elements being as good as large. One centimeter of nerve offers 

 a resistance of about 80,000 ohms and nonpolarizable electrodes have 

 a resistance equal to 700 ohms each. 



Ohm's Law. G. S. Ohm, in 1827, formulated a law: 

 Current strength (amperes) C. = 



E. M. F. = Electromotive force (volts). 

 R = Resistance (ohms). 



But, there are two resistances, so let R stand for internal resist- 

 ance and r for external resistance ; the law will be 



E. M. F. 



C=- 



The ohm, the ampere, and the volt are closely related, and if 

 any two of them are known with reference to any particular electric 

 current, the value of the third may be readily inferred. 

 Currents are measured in amperes, resistances in ohms. 

 Electromotive force is the force which tends to move electricity 

 from a higher to a lower. potential. The unit of electromotive force 

 is the volt, and, therefore, is the measure of electrical pressure. 



Electromotive force is "difference in potential." A volt is that 

 amount of electrical energy which will produce one ampere of current 

 after overcoming one ohm of resistance. 

 Then : 



Volts = amperes X ohms. 

 Amperes = volts -f- ohms. 

 Ohms = volts -f- amperes. 



The small Daniell cell has 4 ohms resistance and a current of 

 1 / 4 ampere. 



"The difference of potential may be compared to the difference 

 of water-level between a reservoir and its distributing pipes. It pro- 

 duces an electromotive force comparable to the force which moves 

 the water from a higher to a lower level. The unit of electrical pres- 

 sure is the volt. The flow through a hydraulic system is measured 





