Final answer:
A governor is the mechanism used to control a prime mover's power output in relation to its load, crucial for the efficient operation of engines and heavy machinery.
Step-by-step explanation:
The mechanism that is affixed to a prime mover to control its power output in proportion to its load is known as a governor. The purpose of a governor is to automatically regulate the fuel supply of the engine, hence controlling the speed based on the load conditions. This ensures the engine operates efficiently and maintains the desired speed without the need for manual adjustment.
For example, in heavy machinery like a backhoe, which uses hydraulic and mechanical lever systems to perform work, a governor would be crucial to ensure that, as the load on the machinery changes, the power output is suitably adjusted to maintain consistent performance.