Figure 56. — Two views of the Von Hefner- 

 Alteneck dynamo witii drum armature as 

 shown at the Vienna Exhibition of 1873. 

 The armature core {s-si, n-nj) is fixed and 

 the armature windings (coiled on abed) 

 rotate. From J. Dredge, Electric Illumination, 

 Londonj n.d. (about 1882), vol. i, p. 278. 



Figure 57. — Wilde's application of a magneto 

 generator to provide the electromagnet field 

 of a second generator. From Philosophical 

 Magazine, 1867. vol. 34, pi. 2. 



Farmer ^^ of the United States in 1865 but their work 

 did not lead to further development. A few years 

 later, in 1867, the principle of self-excitation was 

 simultaneously enunciated by Charles Wheatstone,*^ 

 by S. Alfred Varley,*' and by Werner Siemens (figs. 

 59-61).^' The discoveries of Wheatstone and Siemens 

 were even announced at the same meeting, in London. 

 The basic theory of self-excitation is simple. All 

 iron is magnetized to some extent, however slight it 

 may be, and it is sufficient to induce some current 



in the armature of an electromagnet generator when 

 the armature is rotated between the poles of the 

 electromagnet before any current flows through the 

 electromagnet and before the core of the electro- 

 magnet is "magnetized." If connections are made 

 so that this current passes through the electromagnet, 

 it will increase the magnetic field in which the 

 armature turns, and this in turn increases the induced 

 current, and so on. Under proper conditions, the 

 process will continue until the core of the field magnet 



3= H. Wilde, "On Siemens' and Wheatstone's Magneto- 

 Electric Machines," Proceedings of the Literary and Philosophical 

 Society of Manchester, 1867, vol. 6, pp. 103-107; George B. 

 Prescott, Dynamo-Electricity, New York, 1884, p. 123. 



86 Philosophical Magazine, 1867, ser. 4, vol. 33, pp. 471-474. 



8' British patent 3394 (December 24, 1866); also. Engineering, 

 1877, vol. 24, pp. 322, 348. 



88 Werner .Siemens, "Ueber die Unwandlung von Arbeitskraft 

 in elektrischen Strome ohne Anwendung permanenter Mag- 

 nate," Monatsberichte der Koeniglichen Akademie der Wissenschaften 



zu Berlin, 1867, pp. 55-58; C. William Siemens, "On the Con- 

 version of Dynamical into Electrical Force without the Aid 

 of Permanent Magnetism," Philosophical Magazine, 1867, ser. 

 4, vol. 33, pp. 469-471; British patent 261 (filed January 31, 

 1867); Adolf Thomalen, "Zur Geschichte der Dynamoma- 

 schine," Beitrage zur Geschichte der Technik und Industrie, 1917, 

 vol. 7, pp. 134-168. For an excellent introduction to the 

 early history of the dynamo, see Otto Mahr, Die Entstehung 

 der Dynamomaschine, vol. 5 of Geschichlliche Einzeldarstellungen aus 

 der Elektrotechnik, Berlin, 1941, pp. 129-140. 



378 



BULLETIN 228: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



