502 



ENGINEERING ON THE FARM 



of the two systems, known as multiple series (Fig. 393), gives 

 good results. Gas-engine manufacturers have been using 

 6 volts so long that it may be considered the standard 



Filler pluf, Rubber cover ignitionvoltage. 



Tests have shown 

 that twelve cells con- 

 nected in two series 

 of six each have much 

 more than twice the 

 life that would be 

 obtained from six 

 cells used to the limit 

 of their life and then 

 discarded for the 

 other six. A poor cell 

 connected with good 

 cells may prevent 



Expansion 

 chamber- 



Wooden case 



dealing 



compound — A. . 



Rubber jor- 



Ploles 



E/eclro/yfe 



Hare/rubber 

 supports - 



Mud spaces 

 Fig. 394. 



Cross section of a secondary or 

 storage cell 



satisfactory service, but may be replaced without serious 

 damage having been done to the other cells. 



Secondary cells or storage battery. A storage cell (Fig. 

 394) produces about 2 volts pressure. The amperage 

 depends upon the size and number of plates. Three stor- 

 age cells connected in series deliver the 6 volts commonly 

 used for ignition purposes. They are connected more or 

 less permanently within a wooden case, and there is seldom 

 occasion to disconnect them. The gravity of the electro- 

 lyte approaches 1 .30 when fully charged and may drop as 

 low as 1 . 15 when discharged. If it is allowed to go much 

 below this, there is danger of damage through sulphating 

 of the plates. The specific gravity of the electrolyte is 

 the chief indication of the condition of the cell. While 

 the cell is in operation, some heat is developed and some 

 hydrogen liberated. The result is a lowering of the level 

 of the electrolyte which must be replaced. Distilled water 

 only should be used. Water containing any mineral adds 

 elements which cause local action or short circuits on the 



