Most continuously recording instruments are arranged so as to draw a graph exhibiting the quantity measured by means of cartesian coordinates. The record is on a roll of paper unwound from a drum. If the instrument be some form of meter, the value of the output, represented by ∫ y dx, may be obtained by measuring the area by a planimeter. The records, however, are somewhat bulky, and in this respect a polar graph is preferable. This is drawn by an indicator moving radially on a revolving disc. The peak or maximum of the curve is usually the most important, and this is the part which is represented best on such a polar diagram. Space may be further economised by having more than one convolution on one sheet of paper. Polar output records, however, are not freely used, perhaps because the planimeter does not give directly what is wanted, as it measures ½ ∫ r2-dθ, instead of ∫r dθ.