The throttle valve

The technique for controlling the speed of rotative engines became a subject crucial to their application to cotton-spinning and many other factories. Speed control was something that would react immediately to load changes, in contrast to the expansion engine technique, which involved upgrading or downgrading a steam engine's performance when load situations changed for a more prolonged period.

For controlling the speed of a steam engine a throttle valve was inserted into the steam pipe that supplied the cylinder with steam from the boiler. The throttle valve, which is typically a circular metal plate A with a spindle B fixed across its diameter, allows steam to be admitted into the cylinder of the steam engine more or less freely, allowing rotative engines to be operated at different speeds. The plate A is fitted to an opening in a metal ring CC, through which a spindle B is fitted steam-tight. A spanner is fixed to the spindle B so that it can be turned in either direction.

When the valve plate A is parallel to the ring's outer circumference, it closes the opening almost perfectly. However, when the plane of the valve is at an angle to the ring, it admits more or less steam depending on how much it has opened, which affects how much force is applied to the piston. 

The governer

Steam engines can be controlled with a throttle valve manually for many purposes at the engine keeper's discretion, but when a regular speed is needed, other methods must be used to open and close the throttle valve without the engine keeper's involvement. Mr. Watt experimented with a number of techniques for this purpose before settling on the Governor, which consists of an engine-driven perpendicular axis. Two iron rods suspended in the shape of pendulums carry heavy metal balls E at their lower ends. The iron rods are joint close to the top of the perpendicular axis.