joubert disk

from The Century Dictionary.

• noun a device for the study, by the step-by-step method, of alternating currents and of similar periodic phenomena. It consists of a disk or collar of insulating material with a narrow radial sector of metal. The disk is mounted upon the shaft of the machine or generator for which the type of the wave-form of current is to be determined; or in some cases it is driven synchronously with the machine by means of a separate motor. An adjustable brush, b, makes contact with the metallic sector, a, once in every revolution, and the instantaneous electromotive force for the portion of the cycle corresponding to the position of the brush is measured by means of a condenser and ballistic galvanometer, or by means of some other suitable instrument placed in circuit with the wires + and—. By shifting the brush successively to different points on the periphery a series of measurements is obtained from which a curve representing the cyclic fluctuations in the current generated by the machine may be drawn. This method, which is known as the step-by-step (or point-to-point) method, is applicable only to cases in which the form of the curve is constant and the wave-form repeats itself over and over without sudden changes.