Abstract :
The paper first establishes the equation to the demagnetization curve and then proceeds to discuss the uses of permanent magnets in typical pieces of apparatus. The working of the magnet under these various conditions is considered and the useful part of the magnetic energy is calculated. This introduces a new method by making use of the unit permeance of a circuit, or the permeance as seen from each centimetre cube of the magnet. Certain approximations are made in this calculation and the limitations of the simple theory are then discussed. A method is given of designing a magnet in terms of the constants of the iron circuit. Figures are given for two modern alloys in common use, and curves are plotted for the complete solution to all design problems. The interchangeability of these two alloys is discussed, and it is pointed out that not all the additional energy of the anisotropic alloy can be usefully employed. This is due to the larger curve factor which reduces the recovery when a demagnetizing force is removed. It is shown that the (BH)max value is not the criterion as to the usefulness of a magnet, except in the simplest case, and that it is not necessary for the magnet to work at the (BH)max point.
