STEAM AND GASOLINE ENGINES 



195 



forceful, as the car speeds up. This same valve may be operated 

 by a lever on the steering wheel. 



The mixture of gas and air in the cylinder is ignited by an 

 electric spark. A spark plug is set into the end of the cylinder 

 or just at one side of the end. This bears two metallic points at 

 its inner end between which an electric spark passes when the 

 mixture is properly compressed, and since the mixture is all 

 around the spark the latter ignites it. The electricity is fur- 

 nished either by the storage battery or by a magneto, an electric 



Mixiure to 

 Cylinder 



* Auxiliary 



Air Inlet 



Adi'ustir 



Zen* 



FIG. 79. Diagram of a carburetor 



generator, that is run by a belt or cogwheels attaching to the 

 crank shaft or other moving part. It takes a high-voltage 

 alternating current to send this spark across the gap, a much 

 higher voltage than the battery furnishes, so the current is sent 

 through an induction coil to change the low- voltage direct current 

 of the battery to a high- voltage alternating current. This will be 

 better understood after reading the chapters on electricity (p. 254). 

 The ammeter on the instrument board shows the strength of the 

 current that is being furnished by the battery. A dynamo, 

 power to run which comes from an axle or from the crank shaft, 



