28 PHYSIOLOGICAL PHYSICS. [Chap. in. 



resistances ; so, letting II stand for internal resistance, 

 and putting r for external resistance, the formula is, 



E 



C = 



R+ r' 



So that taking a given cell whose electromotive force 

 is always the same, the strength of current obtained 

 from it will depend on the resistance that it has to 

 overcome, will depend, that is, on the length and thick- 

 ness of the wire along which it is sent, and the nature 

 of the apparatus through which it is conducted. We 

 shall see immediately the bearing of this law of Ohm. 

 Modes of joining cells. There are two ways 

 of joining cells. The positive pole of one cell may be 

 joined to the positive pole of the other, and similarly 

 the two negative poles joined. Where there are 

 several cells, connect all the positive poles to one 



wire ; this wire will be the posi- 

 tive electrode of the battery : 

 then connect all the negative 

 p- i* TVT% r : poles to one wire ; it will be the 



Fig. 15. Mode of joining- r _ 



Cells in " Multiple Arc." negative electrode, I he method 



is called joining in "multiple 



arc." Fig. 15 shows six cells so joined. The effect is 

 just the same as would have been obtained if, instead 

 of taking six cells, a single cell had been taken six times 

 the size of one of them. Now, it has been pointed out 

 (page 26) that the internal resistance of a cell is in- 

 versely proportional to the size of the plates, so that, 

 by multiplying the size of the plates six times, the 

 internal resistance is practically diminished td one- 

 sixth. Increased quantity of current is therefore ob- 

 tained. Thus, neglecting for a moment the external 

 resistance, according to Ohm's law, 



E 6E 



ft'' IT 



