Acetylene lights

Before charging systems and batteries became reliable enough to make electric lighting universal, motorcycles (Harleys included) used acetylene lights. To switch them on (though 'switch' probably isn't the right word), you turned on a tap, which dripped water onto carbide pellets which Prest-o-Lite (press-to-light) reservoir for acetylene lighting which gave off carbide gas, which could be lit. Like other manufacturers, Harley-Davidson moved to electric lighting as soon as possible. The result wasn't searchlight power, but was better than nothing. In fact, a book entitled the ABC of the Motorcycle (published in 1910) states:

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There is only one lamp to use on a motorcycle and that is the acetylene ... Electric lights such as are used on automobiles are out of the question as they require too much extra power over that generated by the magneto.

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The author was wrong: Harley-Davidson was offering electric lights on the 1-model in 1915, and by 1926 all the road-going twins were so equipped.