  The von Neumann interaction between a particle and an apparatus, both of
arbitrary mass, has been considered in the measurement of the position of a
simple harmonic oscillator acted on by an external force. When the measurement
has finite duration, both the motion of the pointer and the oscillator
influence the result of the measurement. Provided that the oscillator is in an
eigenstate of its position at the start of the measurement, the pointer will
indicate the arithmetic average between the initial and final position of the
particle with an added term which depends on the duration of the measurement
and the frequency of the oscillator. This additional term is determined by the
external force which also causes the appearance of a phase factor in the wave
function at the end of the measurement. This phase factor depends on the
average of the initial and final positions of the particle. Furthermore, the
probability that the pointer will indicate a given average value is equal to
the transition probability for the undisturbed free oscillator to experience
the change in position. If the initial state of the pointer is a narrow
wavepacket, then for any initial state of the oscillator, the measurement
yields, approximately, the undisturbed probability distribution for the
position of the free oscillator at the end of the measurement. The transition
probability for the pointer to experience a change in its position has also
been evaluated.
