  A compound pendulum of simple geometry was built from a lightweight rod to
which a pair of masses are clamped, one above and the other below the axis of
rotation. By making the position of the upper mass variable, it was found that
the characteristics of this pendulum can be altered dramatically. A theoretical
model of the instrument is presented, which is in reasonable agreement with
experimental observations. From the model it is shown that isochronism of the
flex-pendulum can be significantly greater than that of the simple pendulum
