25 patents. By 1889 more than 50 companies were manufacturing Otto design engines. Stationary engines Spark plug firing Otto engines were equipped with a number of different mechanism designs to trigger sparking. The Otto is one of the first engines to use a spark plug, which is a device that produces a small electric spark to ignite the fuel charge. This usually consisted of a pivoting trip-arm that briefly grabs a power switch lever and gives it a quick pull. The switch lever is then released and allowed to snap back to its original position in preparation for the next cycle. This system requires an external electric battery, ignition coil, and electric charging system similar to modern automobile engines. Later Otto engines employed a small magneto directly on the engine. Rather than tripping a switch, the spark plug firing arm applies a quick rotation to the magneto rotor, which then snaps back under spring tension. This quick rotation of the magneto coil produces a very brief current flow that fires the spark plug and ignites the fuel. This design has the advantage of requiring no external battery, and is how modern portable gas engines operate, incorporating the magnet portion of the magneto into the flywheel. Modern portable engines excite the magneto with every flywheel rotation, and so use a cam-operated electric switch to prevent plug firing except for the power stroke of the engine (see wasted spark). Engine speed regulation This is a demonstration of how the speed regulation works in the Otto engine. The spinning balls are the centrifugal governor, and as the machine runs slower the small wheel moves to the left, inserting the rod into the nearby roller and pushing it up to trigger the intake of fuel to fire the engine for one revolution. If the machine is under load and still running too slowly, the cam continues to stay inserted and makes the engine fire repeatedly for each ignition cycle. When the engine speed increases, the governor pulls the small wheel to the right and the machine coasts without injecting any fuel, though the spark plug continues to fire with no fuel in the cylinder. This method of speed-control is often referred to as the Hit or Miss method because the engine mis-fires (for lack of fuel-mixture) on those power-strokes where the engine is running faster than the governed speed, but will hit (fire) on