Discussion on “the vibration of telephone diaphragms” (Meyer and Whitehead), Boston, Mass., June 27, 1912. (see proceedings for June, 1912)

George D. Shepardson: Attention is called in the paper to the irregularities in the oscillograph curves at the values 332 cycles and 1292 cycles. I examined that “dimple,” as it is called, with considerable interest, and found two possible explanations for it. In both cases the dimple occurs where the diaphragm is at the greatest distance from the magnet. The first explanation which occurs is that the diaphragm is there in a comparatively weak field where the diaphragm is more free to vibrate in its own natural periods. Another more plausible explanation is that the actuating current in this case is several hundred times greater than the normal current. Now, that means, if we refer to the equation which is given here for the performance of the diaphragm, a very strong m.m.f., due to the current in the coil. At the “dimple,” the current in the negative or demagnetizing direction becomes dominant and neutralizes the m.m.f. due to the permanent magnet; as the current passes the critical value, its pull becomes positive, being proportional to the square of the current, and thus causes the “dimple” at the extreme value. I think this second reason is much more plausible than the first, that is, that it is the m.m.f. of the current overcoming and reversing the permanent field.