GENERAL PHYSIOLOGY OF MUSCLE AND NERVE TISSUE 41 



A concrete idea of the ampere may be gained from the fact that 

 the small-sized Daniell cell produces a current of about \ ampere 

 when the external resistance is reduced to a minimum. 



(2) Resistance is measured in ohms. An ohm is that amount of 

 resistance opposed to the transmission of electric energy by a 

 column of mercury 1 sq. mm. in cross-section and 106.3 cm. in 

 length. For general purposes an ohm resistance is that of a pure 

 silver wire 1 mm. in diameter and 1 metre in length. 



(3) Electromotive force is measured in volts. 



A volt is that amount of electric energy which will produce 1 

 ampere of current after overcoming 1 ohm of resistance. 



"The ohm, the ampere, and the volt are thus closely related, 

 and if any two of them be known with reference to any particular 

 electric circuit or portion of a circuit the value of a third may be 



readily inferred" (Daniell). For if C= ~, theuE=CxR and R=^. 



H 



The same relations may be expressed thus: 1 ampere current 



1 volt E. M. F. 1 volt 



= : , or 1 ampere=- - . 

 1 ohm resistance 1 ohm 



Therefore (1) volts = amperes X ohms; (2) amperes =volts-r- ohms; 

 (3 ) ohms = volts -r- amperes . 



The small Daniell cell has about 1 volt E. M. F. and 4 ohms 

 resistance; the current from such a cell is then equal to approximately 

 J ampere. 



There are numerous other units of measurement used by physicists 

 and electricians, but for our purpose it is not necessary to review these 

 more specialized points. 



VIII. BATTERIES. 



A battery is a group of two or more elements or cells arranged 

 to produce increased or modified effect. If one wishes to use a 

 stronger current than that afforded by one cell, his first thought is 

 to increase the number of cells, or to procure a larger cell. Experi- 

 mentation will show him that it is not a matter of indifference which 

 of these courses to pursue. In the first place, if he attempts to satisfy 

 the conditions he will find that to increase the size of a cell increases 

 the current only when the external resistance is relatively small, 

 and, furthermore, there are practical limitations to the size of a cell, 

 and these may be much within the requirement which the cell must 

 satisfy. It becomes apparent, then, that he who would use electric 

 energy beyond the most limited field must resort to a battery com- 

 posed of a number of cells. The problem which first confronts him 

 is, How shall these cells be arranged? 



1. Appliances. Six Daniell cells; wires; galvanoscope (Fig. 24), 

 composed of a simple magnetic needle mounted over a circle divided 



