"So it does, dad," agreed Bob.  "Well, that's our wireless dynamo.  You will notice that the propeller faces ahead, like the big fellow here. When the airplane is flying, the rush of wind spins the fan at a terrific rate, its axle operates a little dynamo in this torpedo-like case and manufactures electric current.  The current then passes into this small apparatus here with a bulb attached, which regulates the voltage and sends it up to the instruments in a uniform flow, no matter at what speed the airplane may be going."