  Hysteresis, the lag between the force and the response, is often associated
with noisy, jerky motion which have recently been called ``avalanches''. The
interesting question is why the avalanches come in such a variety of sizes:
naively one would expect either all small events or one large one. Power-law
distributions are often seen near transitions, or critical points. We study the
zero temperature random field Ising model as a model for noise and avalanches
in hysteretic systems. Tuning the amount of disorder in the system, we find an
ordinary critical point with avalanches on all length scales. We study this
critical point in 6-epsilon dimensions, and with simulations in 2, 3, 4, 5, 7,
and 9 dimensions with systems as large as 1000^3. The power-law distributions
in principle only occur for a special value of the randomness (the critical
point), but many decades of scaling occur quite far from this special value:
three decades of scaling (a publishable experiment) are obtained a factor of
two away from the critical value. Perhaps many of the power laws observed in
nature are just systems near their critical points?
