GAS, OIL AND ALCOHOL ENGINES 



419 



a very short distance. It differs from the contact-spark 

 circuit in that there must be an induction coil. This 

 coil requires a primary current leading to it from the 

 batteries, and a secondary current leading to the spark 

 points. This latter current has the characteristic of 

 jumping from one point to the other in the form of a 

 spark, thus igniting the charge in the engine (Fig. 308). 

 562. Batteries. In the majority of cases the currents 

 for electric ignitors are furnished by batteries composed 

 of either dry or wet cells. It is very difficult to determine 

 without the aid of proper instruments when a battery 

 has been exhausted to the point where it does not fur- 

 nish sufficient current. Upon trying an exhausted bat- 

 tefy out, it will in all cases give a satisfactory spark. 

 This is due to the fact that batteries when exhausted tend 

 to recover slightly during the rest and are able to furnish 

 current for a few ignitions. Upon starting an engine with 

 an exhausted battery, a 

 few ignitions will take 

 place satisfactorily, but 

 later it will miss fire, 

 due to the weakness of 

 the battery. Often 

 when a battery is be- 

 coming run down and 

 the engine is still run- 

 ning the latter will take 

 in several charges, but 

 no explosion will re- 

 sult; then there is an 

 explosion and a great report from the exhaust. This is 

 because the explosion in the engine ignites the unex- 

 ploded charges which have previously passed through 

 into the exhaust chamber. 



FIG. 309 WIRING SYSTEM FOR BATTERIES 

 AND DYNAMOS 



