Electromagnetic induction and its propagation
By
Originally published in 1885, 1886 and 1887 in "The Electrician":
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XXX. ELECTROMAGNETIC INDUCTION AND ITS PROPAGATION.
The Electrician, 1885-6-7.
[Section I., Jan. 3, 1885, p. 148; II., Jan. 10, p. 178; III., Jan. 24, p. 219; IV., Feb. 21, p. 306; V., March 14, p. 366; VI., April 4, p. 430; VII., April 25, p. 490; VIII., May 15, p. 6 (vol. 15); IX., June 12, p. 73; X., July 3, p. 134; XI., July 17, p. 170; XII., August 7, p. 230; XIII., August 21, p. 270; XIV., August 28, p. 290; XV., September 4, p. 301 ; XVI., October 9, p. 408; XVII., Nov. 13, p. 6 (vol. 16); XVIII., Nov. 27, p. 46; XIX., Dec. 11, p. 86; XX., Dec. 18, 1885, p. 106; XXI., Jan. 1, 1886, p. 146; XXII., Jan. 15, p. 1S6; XXIII., Jan. 22, p. 206; XXIV., March 26, p. 386. The second half of this article is in Vol. II., with the references thereto.]
XXXY. ELECTROMAGNETIC INDUCTION AND ITS PROPAGATION. (Second Half.)
[The Electrician, 1886-7. Section XXV., April 23, 1886, p. 469 ; XXVI., May 14, p. 8 (vol. 17); XXVII., June 11, p. 88 ; XXVIJI., June 25, p. 128 ; XXIX., July 23, p. 212; XXX., August 6, p. 252 ; XXXI., August 20, p. 296; XXXII., August 27, p. 316; XXXIII., November 12, p. 10 (vol. 18); XXXIV., December 24, 1886, p. 143; XXXV., January 14, 1887, p. 211; XXXVI., February 4, p. 281 ; XXXVII., March 11, p. 390; XXXVIII., April 1, p. 457; XXXIXIV., May 13, p. 5 (vol. 19); XXXIXV., May 27, p. 50; XL., June 3, p. 79; XLI., June 17, p. 124; XLII., July 1, p. 163; XLIII., July 15, p. 206; XLIV., August 12, p. 295; XLV., August 26, p. 340; XLVI., October 7, p. 459; XLVII., December 30, 1887, p. 189 (vol. 20).]
XXXVI. SOME NOTES ON THE THEORY OF THE TELEPHONE, AND ON HYSTERESIS.
[The Electrician, Feb. 11, 1887, p. 302.]
As was found in the early days of the telephone, its cores need to be permanently magnetised before it becomes efficient. I refer, of course, to the ordinary magnetic telephone, in which an iron disc is attracted by an electromagnet, which does not differ essentially from a common Morse instrument with a flexible armature, with the important addition that the electromagnet is permanently polarised. The permanent magnetisation may be communicated by a permanent current in the circuit, or, in the usual way, by employing a permanent magnet on whose pole or poles the coils are placed. But the permanent magnetisation, except of the iron disc, is not essential. Thus we may abolish the magnet and core from the telephone, leaving only the coil and disc, and produce the necessary permanent field of force by means of an external magnet suitably placed. The efficiency is then greatly increased by inserting a soft-iron core in the coil. Similarly, we may destroy the efficiency of a complete telephone by an external magnet, or we may increase it, by suitably placing the external magnet so as to, in the first place decrease, and in the second increase the strength of the permanent magnetic field. And if we carry the destruction of the magnetic field by the external magnet so far as to reverse it, and bring it on again strongly enough, we restore the efficiency of the telephone. That is, the permanent polarity may be of either kind. The disc is strongly magnetically attracted in either case, and that is the really essential thing. Most of these facts, if not all, are pretty well known, but it appears to be different as regards their explanation.