I would say both 补偿臂 and 偏心臂 are correct depending on the gadget.

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I would call it a 补偿臂 in a situation like this:

http://shop.store.yahoo.com/cinemasupplies/hojuexofarm4.html

Offset Arms — Offset Arms are a convenient tool when a lighting fixture needs to be placed off center, for the purpose of hiding the stand behind a set wall, or coming over the top of the set. The Baby Extendable Offset Arm terminates in double 5/8” pins and the Junior Extendable Offset Arm terminates in a single 1-1/8” Junior Posi-V-Lock Receiver. Both extend from 18” to 25”.

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It would be 偏心臂 or 偏侧臂 in situations like these:

http://www.neat.com/techinfo/abbe.asp

http://www.crewsonindustries.com/ServiceTypesAvailable.aspx?...

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Note added at 1 hr 5 mins (2005-07-06 14:54:46 GMT)
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translating 补偿臂 back to English, I would suggest extension arm or compensating arm

http://www.hybec.com/product/dermatology/lights.htm

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translating 偏心臂 back to English, I would suggest offset arm, off center arm, or cam arm

http://www.tre-d-eng.it/gripware/home_gripware.html

http://www.jgedelen.com/index.cfm?page=products&category=14