PHYSIOLOGIC APPARATUS. 



70S 



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 positive, + 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 accordance with 

 the resistance will depend the quantity of electricity that a given electro- 

 motive force will press through in a unit of time. The strength of the 



T-4- 



FIG. 354.AN 

 ELECTRIC CELL. 



FIG. 355. Two SIMPLE ELECTRIC CELLS JOINED 

 IN SERIES. C. Copper. Z. Zinc. 



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., the resistance offered by a column of mercury 106.3 cm. l n g 

 and i sq. mm. 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 the ohm, one coul- 

 omb of electricity in one second of time yielding a current strength of^one 

 ampere. 



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



C (current strength) = 



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



Resistance (R) 



Volts 



or Ampers = ^. 



Ohms 



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. 355). If the 

 resistance remains the same the total voltage and current are those of one 

 cell multiplied by the number of cells united. 



The cell as above described cannot maintain a current of constant 

 strength for any length of time, for the following reasons: 



i. The sulphuric acid solution, in consequence of its chemic action, 

 soon becomes nothing more than a saturated solution of zinc sulphate, after 



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