. Advanced lessons in practical physiology for students of medicine. Physiology. 26 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY of electricity between two different points of the conducting path is dependent not onl}^ upon the electromotive force but also upon the resistance of the conductor. A short and thick wu-e 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, i
. Advanced lessons in practical physiology for students of medicine. Physiology. 26 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY of electricity between two different points of the conducting path is dependent not onl}^ upon the electromotive force but also upon the resistance of the conductor. A short and thick wu-e 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 b}'- 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 0° C. The electromotive force of a Daniell cell is about 1 volt and that of an ordinary dry cell 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 = mt. res. and ext. res. ohms Any one of these factors may be determined as follows: volts = amperes X ohms amperes = volts -^- 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
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