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TEXT-BOOK OF PHYSIOLOGY. 



being the copper or the negative element. So long as the elements remain 

 unconnected there is no movement of electricity, no current. 



If the ends of the elements projecting beyond the fluid are connected 

 by a copper wire, a pathway or circuit is established, and a movement of 

 the electricity takes place. As electricity flows from the point of high to 

 the point of low potential, it follows that inside the cell the current flows 

 from the zinc to the copper, and outside the cell from the copper to the. 

 zinc. Such a current is termed a continuous, a galvanic or a voltaic cur- 

 rent. Inasmuch as there is a progressive fall in potential between the 

 highest and lowest points, it follows that any two points in the circuit will 

 exhibit a similar difference of potential. For this reason the projecting 

 end of the copper element is at a higher potential than the projecting end 

 of the zinc element. The end of the copper is, therefore, termed the posi- 

 tive, -J- pole or anode, the end of the zinc the negative, pole or kathode. 

 Electric Units. Owing to the difference of the electric potential in 

 the cell, the electricity leaves the cell under a certain degree of pressure, 

 termed the "electro-motive force." As it passes through the circuit it 

 meets with resistance, the amount of which will depend on the nature of 



the circuit material, its 

 length, and the area of its 

 cross-section. In accord- 

 ance with the resistance 

 will depend the quantity of 

 electricity that a given elec- 

 tro-motive force will press 

 through in a unit of time. 

 The strength of the current 

 will therefore not depend 

 entirely on the electro- 

 motive force, but, rather, 

 on the ratio between the 

 electro-motive force and 

 the resistance. 



For the measurement of electric quantities, a system of units has been 

 devised. The unit of electro-motive force is the volt; the unit of resistance 

 is the ohm, i. e. y the resistance offered by a column of mercury 106.3 cm - 

 long and i sq. mm. hi section at o C.; the unit of quantity is the coulomb; 

 the unit of time is one second. One volt is the electro-motive force which, 

 when steadily applied, will press through a resistance of one ohm, one 

 coulomb of electricity in one second of time yielding a current strength 

 of one ampere. 



This relation may be expressed in the following formula, Ohm's law : 



FIG. 335. Two SIMPLE ELECTRIC CELLS JOINED 

 m SERIES. C. Copper. Z. Zinc. 



Electro-motive force (E. M. F.) A Volt 



C (current strength) = - -^5 = - or Ampere = 



Resistance (R) niw 



Ohm 



In practical work it is often necessary to increase the strength of the 

 current. This is done by uniting two or more cells in series, i. e., uniting 

 the copper of one cell to the zinc of a second, and so on (Fig. 335). If the 

 resistance remains the same the total voltage is that of one cell multi- 

 plied by the number of cells united. 



