  Galactic cosmic ray (CR) acceleration to the knee in the spectrum at a few
PeV is only possible if the magnetic field ahead of a supernova remnant (SNR)
shock is strongly amplified by CR escaping the SNR. A model formulated in terms
of the electric charge carried by escaping CR predicts the maximum CR energy
and the energy spectrum of CR released into the surrounding medium. We find
that historical SNR such as Cas A, Tycho and Kepler may be expanding too slowly
to accelerate CR to the knee at the present time.
