470 



FARM MOTORS 



624. Cells. If a strip of copper be connected to one end 

 of a strip of zinc and the free ends of the two metals 

 be immersed in dilute sulphuric acid (Fig. 352) a cur- 

 rent of electricity will manifest itself in the wire. If the 

 circuit is broken and the plates carefully watched, 

 bubbles will be seen to collect on the zinc plate and none 

 on the copper. As soon, however, as the circuit is com- 

 pleted again a current will be noticed, also a great 

 number of bubbles will appear about the copper plate. 

 These last bubbles are bubbles of hydrogen and always 

 appear when a current is being produced. The bubbles 

 which form about the zinc are also of hydrogen, but they 

 are caused by the zinc being impure and by a current 

 starting up between these particles of impurities and the 

 particles of zinc. This action is detrimental to the cell 

 and should be stopped by covering the zinc with mercury. 



By permitting the current 

 of this cell to run for some 

 time it will be noticed that 

 the zinc is being gradually 

 eaten away, and that the 

 copper plate does not 

 change. From this it is 

 learned that when the cur- 

 rent of a simple cell is 

 formed the zinc is eaten 

 away and hydrogen collects 

 on the copper. The cur- 

 rent passes out from the 

 FIG. 352 CELL copper plate and in on the 



zinc. In other words, the copper plate is the positive ter- 

 minal and the zinc is the negative. 



625. Polarization. After the current has run for some 

 time in the cell as previously described the strength will 



