358 



AGRICULTURAL ENGINEERING 



Fig. 229. A 

 spark plug in 

 section, show- 

 ing construc- 

 tion. 



that the trouble lies elsewhere than in the ignition system. 

 The Batteries. Any form of electric ignition requires a 

 source of electricity. One of the most general forms on the 

 market is the dry-cell battery. It represents, 

 perhaps, the cheapest source of electricity, as 

 far as first cost is concerned. When the cells are 

 able to furnish a sufficient quantity of electricity, 

 they are very satisfactory. One of the most 

 perplexing features of the use of dry-cell bat- 

 teries is the matter of determining when the 

 cells are exhausted, as there is no change in 

 the outside appearance. 

 There are instruments, known 

 as ammeters, which enable 

 one to determine how much current a dry 

 cell will furnish; and where many dry cells 

 are used, this instrument should always 

 be on hand to detect exhausted cells. If 

 an instrument is not 

 available, the strength 

 of the cells must be 

 judged from the size 

 and character of the sparks produced when 

 tested. 



Storage batteries make a very satis- 

 factory source of electric current for igni- 

 tion, but provision must be at hand for 

 recharging when they become exhausted. 

 Fig. 231. An oscii- Magnetos and Dynamos. Perhaps the 



lating magneto on . » n i . • 



demonstration stand, most satisfactory source oi electric current 

 for gasoline engine ignition is the magneto or dynamo, 

 which is a small instrument for making electricity by me- 

 chanical means. Indications are that it will be only a 



Fis 



230. A storage 

 battery. 



