130 Prof. S. P. Thompson on the Source 



theory appears to have been started by Professor Bell himself* 

 in order to account for the transmission of speech by instru- 

 ments having very thick iron diaphragms, and by the instru- 

 ments having no diaphragms at all. This view has been also 

 upheld by the Comte Du Moncel in several communications 

 to the learned societies of France. The other view appears to 

 have been first distinctly put forward by Mr. W. H. Preece, 

 in introducing the telephone to the British Association at 

 Plymouth in 1877 f ; and it has for its most vigorous suppor- 

 ters M. A. NiaudetJ and Colonel Navez, the latter of whom 

 has replied more than once to points raised by M. Du Moncel. 

 It is a view which appears also to be supported by the recent 

 experiments of Professor Hughes. 



The evidence now to be adduced, though not absolutely 

 conclusive on the point at issue, opens out several fresh points 

 of interest. It consists, in brief, of the results obtained by 

 applying to the field of the telephone the experimental method 

 of studying the so-called lines of force, originally due to Gil- 

 bert, and developed by Faraday. The details of the method 

 followed by the present writer are identical with those de- 

 scribed in his communication of June 23, 1878, " On Magnetic 

 Figures," &c, and which consists in fixing permanently onto 

 glass plates the figures obtained by iron-filings. 



The figures obtained by means of iron-filings were resorted 

 to with the view of ascertaining whether the changes in the 

 magnetic field of the telephone were sufficiently marked to 

 account for the alleged motions executed by the iron diaphragm, 

 or whether they were such as to give any support to the mo- 

 lecular hypothesis. 



The first step was to investigate the field of a bar-magnet 

 when one pole was placed near a thin iron diaphragm. 



It w r as known at the outset that a thin plate of magnetic 

 matter might be magnetized in an enormous variety of ways. 

 The magnetism might be distributed on the two faces, or in 

 the manner known as lamellar ; or, instead, any two points in 

 the disk might be taken as conjugate poles ; or any number 

 of poles might be introduced ; or, as in the magnets of M. 

 Duter, the magnetization might be radially distributed, the 

 central portion having one polarity, the other polarity existing 

 all round the circumference. De Haldat showed that a variety 

 of irregular magnetizations might be produced by touching 



* A. Graham Bell, " Researches in Electric Telephony," Journ. Soc. 

 Telegr. Eng. 1878, p. 414, vol. vi. 



t Rep. Brit. Assoc. 1877, Plymouth, p. 13, W. II. Preece, C.E., "On 

 the Telephone." 



| Telephones et Phonoyraphes, p. 92. 



