The angular momentum carried by a plane wave with a spiral phase singularity exp(imφ) is a phenomenon which has been observed qualitatively in systems such as the "optical wrench". From electromagnetic wave theory, the rate of transport of angular momentum L of a spiral wave is related to the Poynting vector S by L=mS/ω,  and in quantum mechanics the simple equivalent is that a photon carries angular momentum l= mħ. Although these results are well-known, they have not been directly validated. We constructed an experiment in which angular momentum from a spiral wave is absorbed by a magnetic block levitated above a diamagnetic plate of graphite. By measuring the rate of rotation induced in the block we have confirmed the above results, finding that l=(0.97±0.3)mħ.