For balancing a single disturbing mass, what is the minimum number of balance masses required to be introduced in a plane parallel to the plane of rotation of the disturbing mass?

At a certain speed, revolving shafts tend to vibrate violently in the transverse direction. This speed is known as:

In the dynamic balancing of machines, which of the following is NOT a recognized method of measuring the magnitude and location of correction weight for a two-plane correction?