i88 



SCIENCE. 



[Vol. XVII. No. 426 



SCIENCE; 



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CHANGE OF FORM AFFECTING A MAGNETIC FIELD.' 



Hitherto the study of a magnetic field has been the study of 

 the so-called lines of force radiating from the poles of magnets, 

 either electro or permanent; and, so far as magnetism has been 

 utilized in the arts, the changes in this external field have been 

 brought about by the movements of an armature, having for its 

 function to determine the direction and consequent density of the 

 field. Such is the case in the instruments used in the telegraph, 

 the telephone, in dynamos, and in motors Sometimes conduct- 

 ing wires are so mounted in the field that their movement gives 

 rise to electric currents, which signifies that the energy producing 

 the tension in the field is absorbed in some measure by the mov- 

 ing wires, and is transformed into an electric current. In each 

 of these cases the magnet producing the field is stationary ; that 

 is, changes in the magnetic field produced by it are due to a 

 motion external to the magnet itself, and may be that of an 

 armature, of a moving wire, or of its own bodily change of 

 position, — a kind which is comparable with what is called external 

 motion in thermo dynamics, to distinguish it from internal 

 motions, or such as take place when the body changes its form. 

 So far as I am aware, no study has been made Of the effect of 

 changing the form of a magnetic body on its field, or of the re- 

 action upon itself of its magnetic condition due to a periodic 

 change of form. Of course, it has been known for a long time 

 that the form of the magnetic field depended upon theform of the 

 magnet itself. For a straight bar magnet, this field is familiarly 

 known by the arrangement of iron filings forming curved lines 

 from each pole re-entering the opposite pole. When the iron is 

 bent into a U-form, or hoiseshoe magnet, the field is mostly con- 

 tracted to the space between the poles. These forms of magnets 

 have been permanent ones for the purpose for which the magnet 

 was made. 



In the case of induction-coils, whether of one form or another, 

 the magnetic change produced by it has been and is due to the 

 electric change produced upon it by an electric circuit provided 

 with intermittent or alternating currents. 



Within a few years, attention has been called to the nature of 

 the external field as being a part of what is now known as the 



' Paper presented Jan. 14, 1891, by A. Emerson Dolbear, to the American 

 .Academy of Arts and Sciences, Boston. 



magnetic circuit, which consists of these rings or closed circuits 

 of lines of force, all originating in the iron part of the circuit, and 

 for conducting which iron is by far the best. The poles of the 

 magnet are simply the parts of the iron where the lines enter and 

 leave, and they may be in any place. Usually they are at the 

 ends of the iron, but not necessarily so. Whenever iron is placed 

 in the magnetic field, these lines crowd into it, as it is a mucin 

 better conductor than the ether. When the iron is made into a. 

 ring form and then masinetized, there is no external polarity, anrt 

 consequently no external field, provided that the iron has sufficient 

 conducting cross-section at every part. 



The following experiments have been tried, to determine what 

 effects, if any, are produced upon a magnetic field by changing 

 the form of the magnet. It was thought at first, that if a helix 

 was coiled into a circle and a current was present in it, changes 

 in its form would produce corresponding changes in the magnetic 

 field external to the coil, especially noticeable if a flexible iron 

 ring was enclosed in the helix so as to condense the magnetic field. 

 This was put to the test in the following manner 



I. A coil similar to the one described above, but containing ai 

 solid ring of iron about eight inches in diameter and an inch thick, 

 had its coil put in circuit with a reflecting galvanometer of low 

 resistance, and at such a distance from it that magnetic fields ex- 

 ternal to its circuit could not act upon it. Another coil made 

 about a flexible ring of iron wire was put in circuit with a battery, 

 so as to magnetize the ring strongly. Then, with one ring 

 parallel to the other, the flexible one was made suddenly to assume 

 an elliptical form. Each such change in form, from one ellipse 

 to another at right angles to it, gave a deflection of the needle to 

 the right or left, and uniformly for a given phase of change. It 

 was al.'iO observed that the direction of the deflection was reversed 

 when the flexible ring was turned the other side up. 



II. The same flexible ring, used in the same way, but without 

 the current through it, gave substantially the same results. Of 

 course, the ring was permanently magnetized, and the change 

 might have been inferred. 



III. As the same kind of motion, due to change of form, is taking 

 place when a ring is vibrating at its harmonic rate, producing 

 what we call sound-vibrations, it was thought probable that a 

 magnetized ring, having a coil of wire about it in connection with 

 a telephone, would set up vibratory currents when it was struck; 

 and this was found to be true, for, when the coil containing the 

 heavy iron core was put in circuit with a telephone in another 

 room, the sound of the stroke and the pitch of the ring could 

 plainly be heard. In the first case, the number of turns of wire 

 was small, perhaps fifty or thereabouts. I therefore had two 

 larger rings made, each about one foot in diameter and half an. 

 inch thick. 



IV. One of these was wound with six or seven hundred turns 

 of No. 32 wire. Before it was magnetized, it was connected with 

 the telephone, and tested for its magnetic condition by striking. 

 The ring could plainly be heard, which showed that it had some 

 degree of magnetism. 



v. Then about two hundred turns of coarse wire were woundl 

 upon it, and a strong current sent through it to magnetize it. 

 After this magnetizing coil had been removed, the ring was againi 

 tested as in IV. The sound was very much louder. Indeed, the 

 telephone could be held a foot from the ear and be heard. 



VI. With the ring in V. still in circuit, the companion ring, 

 without any wire upon it, was brought near it and struck. The- 

 sound was easily heard in the telephone circuit. 



VII. This second ring was now magnetized in the same way as 

 the first, when the magnetizing helix was removed, and experi- 

 ment VI. repeated. The sound was very much louder. 



VIII. The ring was now struck and moved away from the first, 

 ring by stages of an inch or two at a time. It was found possible 

 to hear its pitch in the second circuit, when it was a yard or more 

 away from it. 



IX As the pitch of the two rings was not quite the same, the 

 higher one was loaded so as to bring them to unison. The sound 

 was then louder and more persistent than before. This gave evi- 

 dence that it was a case of sympathetic vibration, while the former 

 were forced vibrations. 



