Richardson's handbook of projection (1930)

1110 HANDBOOK OF PROJECTION FOR shoe) magnet, in such manner that each end of the core is close to, but not in contact with, one end of the U. When used thus the core is called an "armature," and most of the magnetism produced by the U passes through it, and, as you will readily understand, the closer the armature ends are to the U tips, the greater will be the amount of magnetic flow, and vice versa. You are told on page 987 that in following the groove in a record the needle does not bob up and down through a series of indentations in the bottom of the groove, as many suppose, but that the sound is carried in the undulations of the sides of the record groove, which cause the needle to sway slightly from side to side. The movement of the needle and the law above cited are the two things which make the Western Electric disc record pick-up possible, for such a coil and armature are located in a magnetic field, substantially as described, and the distance of the armature from the magnet poles is controlled by the movement of the needle as it travels through the groove-side undulations. It therefore is not difficult to understand that as the needle moves the armature back and forth, E M F will be generated in the coil, which will exactly correspond to the groove undulations, and since those undulations correspond exactly to the sound waves which produced them, it follows that we will have E M F generated in the coil which will correspond in amount or power precisely to the sound waves. In other words, we will have the sound waves thus exactly reproduced in the form of electric voltage, or "current." In considering this, gentlemen, you must try to realize the enormous rapidity of electrical action — a difficult thing for any but the engineer to do — and understand that while