26 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



of electricity between two different points of the conducting path is 

 dependent not only upon the electromotive force but also upon the 

 resistance of the conductor. A short and thick wire possesses less 

 resistance than a long and thin wire; hence, provided that the electro- 

 motive force remains constant, the flow of electricity will be greater 

 in the first instance. Besides this external resistance encountered by 

 the current in its passage from the copper to the zinc, it is also opposed 

 by an internal resistance resident in the constituents of the battery. 

 In the latter case, the resistance is the less the larger the surface of the 

 plates. 



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 volume of mercury 1 mm. in area and 1063 mm. in 

 length at C. The electromotive force of a Daniell cell is about 

 1 volt and that of an ordinary dry cell 1.5 volt. 



The relationship existing between these different factors is expressed 

 by Ohm's law, in accordance with which the 



electrom. force volts 



current strength = - or amperes = - 



int. res. and ext. res. ohms 



Any one of these factors may be determined as follows : 

 volts = amperes X ohms 

 amperes = volts H- ohms 

 ohms = volts -^ amperes 



FIG. 3. MERCURY KEY. 



2. The Simple Key. Living substance may be stimulated with an 

 electric current by simply touching it with the ends of the loose wires 

 leading out from the poles of a battery. A better way, however, is to 

 leave the wires in firm contact with the living substance and to stimu- 

 late it by making and breaking the current by means of a key. Three 

 kinds of keys are commonly used in the laboratory, namely, mercury, 

 friction (DuBois-Reymond), and automatic keys. 



The mercury key consists of a round wooden base weighted with 

 iron. The center of its upper surface is depressed for the recep- 



