Theory of Machine
A Hartnell governor has its controlling force (Fc) given by Fc = ar + b, where r is the radius of rotation and a and b are constants. The governor becomes isochronous when

a is +ve and b = 0
a = 0 and b is +ve
a is +ve and b is also +ve
a is +ve and b is -ve

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Theory of Machine
Lower pairs are those which have

Elements of pairs not held together mechanically
Two elements that permit relative motion
Point or line contact between the two elements when in motion
Surface contact between the two elements when in motion

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