The exact equations for the behaviour of a tethered float radiation fo
rce device are derived and shown to be analytically insoluble at the p
resent time. In the limiting case of vertical supporting chains, the a
nalysis becomes trivial and the device has a linear response. The cons
truction and evaluation of such a device for assessing medical physiot
herapy equipment is described and the results of a pilot study on ten
clinical machines is presented. The poor relation between their actual
and their nominal behaviour indicates the need for closer control of
both manufacture and performance in the field.