The movement consists of a bimetallic spiral which is expanded by the heat caused by the current circulating in it, driving the pointer. A second spiral, mounted in opposition, compensates the effect of the ambient temperature.
Due to its long response time, shor interval current peaks are not registered and the instrument indicates by a black pointer the mean of the r.m.s. current in a specified period of time (15 minutes). The maximum indication is registered by the position of the red pointer, dragged by the black one. By means of a sealable knob it is possible to resetthe red pointer to the position of the black one in order to make a new reading. The maximum demamd meters are used to monitor lias in feeders, transformers and electrical installations in general.