32 PHYSIOLOGICAL PHYSICS. [Chap. in. 



given, then n 20, that is, 5?^ = 20m ; there- 



w 



fore n 4m. That is, the number of groups is equal 

 to four times the number of cells in each group. But 

 there are thirty-six cells in all, or the number of 

 groups multiplied by the number of cells in each 

 group = 36, i.e. n x m = 36. But, as shown, n = 

 4m, therefore 4m X m 36, or 4m 2 = 36, therefore 

 m 2 = 9, i.e. m = 3, and n = 4m, i.e. 12. Therefore, 

 to get the strongest current the number of groups (?i) 

 should be 12, and the number of cells (m) in each 



group should be three. (Fig. 17 

 shows on one side six cells arranged 

 in two series, and on the other the 

 same number arranged in three 

 series.) 



Divided eircinifs. In Fi<r. 18 



C* 



there is shown a main circuit, from 

 which at the point A two secondary 



c i rcuits arise to j oin tne main I" 16 



in Groups. again at B. Now the electricity 



flowing along from P will divide at 

 the point A, part will continue along the straight course 

 to B, and part will pass by r 2 and r s . The currents flow- 

 ing in r 2 r s are called the " derived " currents, A and 

 B being the " points of derivation." Supposing the 

 wires r l r 2 and r s to be of the same material, length, 

 and cross-section, then they would offer precisely the 

 same amount of resistance to the passage of the cur- 

 rent. The result would be that the same amount of cur- 

 rent would flow along the three wires, but that the total 

 quantity of current flowing between A and B would be 

 greater than if there had been only a single wire r l be- 

 tween A and B. If, however, for r } a wire of the same 

 length, but treble its thickness, were substituted, then 

 the intensity would be the same as passes by the three 

 wires. In other words, the presence of the three wires 



