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I have a motorcycle headlamp bulb on my desk here. I notice the bulb has a rating of "60/55W" and has three connectors instead of the usual two.

Evidently these correspond to the high/low beam setting respectively but how does it work in practice? Are there two separate filaments within the bulb - one 55W and one 60W - that can be turned on either individually or together?

Thanks.

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    \$\begingroup\$ Similar set-up is used on some vehicles for rear lights. One filament for position light and one for stop light. \$\endgroup\$ – AndrejaKo Jun 14 '11 at 21:12
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    \$\begingroup\$ I noticed this on my bike rack today! I was interested in why but didn't have the time to form the question. \$\endgroup\$ – Dean Jun 14 '11 at 21:38
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It simply has two filaments, you should be able to see both of them. The lower power one is for the dipped beam, and is positioned to give a low beam.

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    \$\begingroup\$ In my old car, it was one or the other. I could light both at the same time by holding the high-beam lever back, though. \$\endgroup\$ – endolith Jun 14 '11 at 21:41

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