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

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Theory of Machine
The contact ratio is given by

(Length of the arc of approach)/cosφ
(Length of the arc of contact)/(Circular pitch)
(Length of path of recess)/(Circular pitch)
(Length of the path of approach)/(Circular pitch)

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