The zero circle and circle of correction are defined as the circle round the circumference of which if the tracing point is moved, the wheel will simply side on the paper without any change in reading. This is possible when tracing arm is held in such a position relative to the anchor arm that the plane of the roller passes through the anchor point i.e. the line joining the anchor point and the wheel is at right angles to line joining the tracing point and wheel.

The area of zero circles can be found by following method:

1. When the constant C and M are known

Area of zero circle M x C

2. Area of zero circle (R0) = π [( L^2)+2Lf + (R^2)]

Where,

L=the length of the tracing arm from hinge to the tracing point

d= the distance from hinge to the wheel

R0=the radius of zero circle = ((L^2)±2Lf + (R^2 ))

R= the length of anchor arm from hinge to the anchor point.