272 PROCEEDINGS OF THE AMERICAN ACADEMY 



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 inter- 

 mittent 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 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 

 80. Whenever iron is placed in the magnetic field, these lines crowd 

 into it, as it is a much better conductor than the ether. "When the 

 iron is made into a ring form and then magnetized, there is no ex- 

 ternal polarity, and 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 a 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 external 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 de- 

 flection of the needle to the right or left, and uniformly for a given 

 phase of change. It was also observed that the direction of the de- 

 flection 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. 



