demonstrating a limit to the Magnetiz ability of Iron. 307 



in a bar of soft iron. This fact^ which_, although at variance 

 with the views of Poisson, has been arrived at a priori by Prof. 

 Thomson from theoretical considerations*,, has been confirmed 

 by the important researches of Feilitzschf, Gartenhauser, and es- 

 pecially of Miillert, whose numerous experiments appear to have 

 been made with very great care^ and to have dispelled all doubt 

 as to the existence of such a limit. The interest which these 

 comparatively recent researches have so justly excited has en- 

 couraged me to hope that you will consent to reprint a few short 

 papers, in which the discoveries of the above-named philosophers 

 are confirmed, but which have hitherto only received publication 

 in a periodical which seems not to have been widely circulated, 

 I have made a few alterations, but with a view to explain, not in 

 the smallest degree to alter the meaning of the original. 

 I have the honour to remain. Gentlemen, 



Yours very respectfully, 



J. P. Joule. 



On the Use of Electro-magnets made of Iron Wire for the Electro- 

 magnetic Engine, By J. P. Joule, Esq. Communicated in 

 . a Letter to the late Mr. Sturi^eon§. 



Dear Sir, H Salford, March 27, 1839. 



In my last letter I gave you an account of some experiments 

 which were intended to prove that electro-magnets made of iron 

 wire are the most suitable for the electro -magnetic engine. In 

 those experiments round wire was used j and it was my opinion 

 that the wire magnets were put in a disadvantageous position, 

 in consequence of the interstices between the wires. I have since 

 confirmed my views on this subject by the following experiment: — 

 I constructed two magnets. The first consisted of sixteen 

 pieces of square iron wire, each jyth of an inch square and 7 

 inches long, bound very tightly together so as to form a solid 

 mass, whose transverse section was y^ths of an inch square ; it 

 was enveloped by a ribbon of cotton, and wound with sixteen feet 

 of covered copper wire, of y^^th inch diameter. The second 

 was made of solid iron, but was in every other respect precisely 

 like the first. These magnets were fitted to the apparatus used 

 in my former experiments, and care was taken to make the fric- 

 tion of the pivots equal in each. The mean of several experi- 



* Phil. Mag., vol. xxxvii. p. 252. 

 , t " Electromagnetismus in weiches Eisen und iiber den Sattigungspunkt 

 derselben." — PoggendorfF's Annalen, 1850. 



X Ueber den Sattigungspunkt der Electromagnetismus von J. Miiller 

 in Freiburg.— Fogg. Ann., Feb. 1851, vol. Ixxxii. p. 181. 



§ Annals of Electricity, vol. iv. p. 68. '^^^':'*' '*i ' 



