The instrument is constructed to drive some control, such as the index of a temperature controller, through a pre-set programme over a period of several weeks. The programme is approximated by a series of phases of uniform mean rate of rotation of a driven shaft. Ten speeds, forward and reverse, over a ratio 30: 1, are available, and the duration of each of six phases can lie between 10 minutes and 12 days. The speeds are determined by a relay-uniselector circuit which transmits a fraction of regular 1 minute impulses to the driving motor. This turns the driven shaft through half a revolution upon the receipt of each impulse.