This cable rotates the governor pulley at a speed directly proportional to the speed of the car. In the event of excessive car speed, the governor uses another cable to activate the emergency brake jaws which grip the guide rails and slow the car to a stop.
http://www.madehow.com/Volume-2/Elevator.html#b
http://www.otisworldwide.com/pdf/AboutElevators.pdf

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Note added at 40分 (2012-07-30 23:58:59 GMT)
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or:wedge
1. The governor must be roped up and the car's safeties (e.g. "wedges" or "safety jaws" which act as rail brakes which grab the elevator's guide rails) made operational prior to hoisting the car sling. In the event of a safety failure (e.g. failure of a support sling(s)) or a drop in the platform the governor is designed to mechanically actuate the car's safeties, which are part of the car sling and platform. When the safeties engage, the car stops prior to falling to the bottom of the shaft, thus preventing a "free fall" of the car sling.
http://www.osha.gov/dts/hib/hib_data/hib19950622.html

・非常止め装置
吊りロープ切断時または速度が超過し調速機が動作した時等に、かごを停止 させるための装置。一般にガイドレールまたは安全索に働き、落下の力でくさ び作用を利用してウェッジ等を噛込ませて制動力を得るもの。 極く低速のものでは、ラチェットと爪や係止ピンによるもの、調速機の遠心力を 直接摩擦ブレーキに応用するもの等があるが、
http://www.jpo.go.jp/shiryou/s_sonota/pdf/pat_guidebook/04_e...