We consider a Brownian ratchet system which circulates fast and randomly be
tween two thermal sources with different temperatures. The steady current o
f the system is evaluated by the Langevin simulations. An analytical expres
sion of the current for fast exchange of the noises is obtained. The featur
e of temperature-dependent current is discussed. The results show that the
current for randomly exchanging two white noises is larger than that of per
iodically changing temperature under the same parameters. (C) 1999 Elsevier
Science B.V. All rights reserved.