[Continued from SUPPLEMENT, No. 786, page 12558.] THE ELECTROMAGNET. [Footnote: Lectures delivered before the Society of Arts, London, 1890. From the Journal of the Society.] BY PROFESSOR SILVANUS P. THOMPSON, D. SC., B.A., M.I.E.E. III. RESEARCHES OF PROFESSOR HUGHES. [Illustration: FIG. 51.--HUGHES' ELECTROMAGNET.] His object was to find out the best form of electromagnet, the best distance between the poles, and the best form of armature for the rapid work required in Hughes' printing telegraphs. One word about Hughes' magnets. This diagram (Fig. 51) shows the form of the well known Hughes' electromagnet. I feel almost ashamed to say those words "well known," because on the Continent everybody knows what you mean by a Hughes' electromagnet. In England scarcely anyone knows what you mean. Englishmen do not even know that Professor Hughes has invented a special form of electromagnet. Hughes' special form is this: A permanent steel magnet, generally a compound one, having soft iron pole pieces, and a couple of coils on the pole pieces only. As I have to speak of Hughes' special contrivance among the mechanisms that will occupy our attention later on, I only now refer to this magnet in one particular. If you wish a magnet to work rapidly, you will secure the most rapid action, not when the coils are distributed all along, but when they are heaped up near, not necessarily entirely on, the poles. Hughes made a number of researches to find out what the right length and thickness of these pole pieces should be. It was found an advantage not to use too thin pole pieces, otherwise the magnetism from the permanent magnet did not pass through the iron without considerable reluctance, being choked by insufficiency of section: also not to use too thick pieces, otherwise they presented too much surface for leakage across from one to the other. Eventually a particular length was settled upon, in proportion about six times the diameter, or rather longer. In the further researches that Hughes made he used a magnet of shorter form, not shown here, more like those employed in relays, and with an armature from 2 to 3 millimeters thick, 1 centimeter wide and 5 centimeters long. The poles were turned over at the top toward one another. Hughes tried whether there was any advantage in making those poles approach one another, and whether there was any advantage in having as long an armature as 5 centimeters. He tried all the different kinds, and plotted out the results of observations in curves, which could be compared and studied. His object was to ascertain the conditions which would give the strongest pull, not with a steady current, but with such currents as were required for operating his printing telegraph instruments; currents which lasted but one to twenty hundredths of a second. He found it was decidedly an advantage to shorten the length of the armature, so that it did not protrude far over the poles. In fact, he