The capacity effect and natural free period of a direct-current armature

The analogy between the absorption of energy by a coil free to rotate in a constant magnetic field and the charging of an electrostatic condenser was pointed out by Swinburne in 1902, and was again stated by Kapp in developing the theory of his vibrator. The phenomenon of instability of direct-current shunt motors with interpoles was treated by Siebert, Wagner, Rosenberg and others, while the instability of differntially compounded motors was dealt with by Greve and Andrade. In the present paper, the correspondence between electrostatic and ¿dynamical¿ capacities is re-stated in a some-what different form, and the theory of the instability of compounded motors is developed as an extension. A series of experiments is described, the results of which are cited as confirmation of the results obtained by the mathematical analysis, and, in the appendix, details of the solution of certain of the equations are given.