  The chameleon scalar field is a matter-coupled dark energy candidate whose
nonlinear self-interaction partially screens its fifth force at laboratory
scales. Nevertheless, small-scale experiments such as the torsion pendulum can
provide powerful constraints on chameleon models. Here we develop a simple
approximation for computing chameleon fifth forces in torsion pendulum
experiments such as Eot-Wash. We show that our approximation agrees well with
published constraints on the quartic chameleon, and we use it to extend these
constraints to a much wider range of models. Finally, we forecast the
constraints which will result from the next-generation Eot-Wash experiment, and
show that this experiment will exclude a wide range of quantum-stable models.
