  Colloidal suspensions of ferromagnetic nano-particles, so-called ferrofluids,
are shown to be suitable systems to demonstrate and investigate thermal ratchet
behavior: By rectifying thermal fluctuations, angular momentum is transferred
to a resting ferrofluid from an oscillating magnetic field without net rotating
component. Via viscous coupling the noise driven rotation of the microscopic
ferromagnetic grains is transmitted to the carrier liquid to yield a
macroscopic torque. For a simple setup we analyze the rotation of the
ferrofluid theoretically and show that the results are compatible with the
outcome of a simple demonstration experiment.
