An electromechanical instrument has been developed, which adds, subtracts, multiplies, or divides the ordinates of two curves in Cartesian co-ordinates, plotting the resulting locus. This uses the principle of the Wheatstone bridge, and is semiautomatic; the curves are followed manually, but a phase-sensitive servo system keeps the bridge in balance. Attachments are provided for determining the area under the resulting curve, and also for the performance of certain logarithmic operations of interest in photometry. In the ordinary use of the computer, the accuracy is about 0.3% of full scale or better.