308 ELEMENTARY LESSONS ON [CHAP. vi. 



plate, then through a connecting wire or screw to the next 

 cell, through its liquid, through the connecting screws and 

 liquids of the rest of the cells, then through a wire to a 

 galvanometer, then through the coils of the galvanometer, 

 then perhaps through an electrolytic cell, and finally 

 through a return wire to the zinc pole of the battery. In 

 this case there are a number of separate electromotive-forces 

 all tending to produce a flow, and a number of different 

 resistances, each impeding the flow and adding to the 

 total resistance. If in such a case we knew the separate 

 values of all the different electromotive-forces and all the 

 different resistances we could calculate what the current 

 would be, for it would have the value, 



Total electromotive-force 

 Total resistance 



If any one of the cells were set wrong way round its 

 electromotive-force would oppose that of the other cells ; 

 an opposing electromotive-force must therefore be sub- 

 tracted, or reckoned as negative in the algebraic sum. 

 The "polarisation" (Arts. 163 and 413) which occurs 

 in battery cells and in electrolytic cells after working for 

 some time is an opposing electromotive - force, and 

 diminishes the total of the electromotive -forces in the 

 circuit. So, also, the induced back-current which is set 

 up when a current from a battery drives a magneto- 

 electric engine (Art. 377) reduces the strength of the 

 working current. 



346. Conductivity and Resistance. The term 

 conductivity is sometimes used as the inverse of 



resistance ; and the reciprocal represents the con- 

 ductivity of a conductor whose resistance is r ohms. In 

 practice, however, it is more usual to speak of the 

 resistances of conductors than of their conductivities. 



