  The effects of the measurement apparatus on quantum coherence are studied by
considering a purely dephasing model of a qubit. The initial state is prepared
from a thermal state of the whole system by performing a nonselective
measurement on the qubit. The magnitude of the initial postmeasurement
coherence is bounded by the value $1/2$, which is realized with special
measurement schemes and in the low-temperature limit. The magnitude of
coherence identically vanishes, increases or decreases with approximately
constant velocity over a determined short time scale, according to the choice
of the preparation measurement. The maximization of the short-time increasing
or decreasing velocity is favored by the choice of further special measurement
schemes and the high-temperature limit. The measurement apparatus allows to
manipulate quantum coherence of the qubit over short times via nonselective
preparation measurements.
