  We present an exact derivation of a process in which a microscopic measured
system interacts with "heat bath" and pointer modes of a measuring device, via
a linear coupling involving Hermitian operator $\Lambda$ of the system. In the
limit of strong interaction with these modes, over a small time interval, we
show that the measured system and the "pointer" part of the measuring device
evolve into a statistical mixture of direct-product states such that the system
is in each eigenstate of $\Lambda$ with the correct quantum-mechanical
probability, whereas the expectation values of pointer-space operators retain
amplified information of the system's eigenstate.
