276 Mr. W. Duddell [March 16, 



are not wholly to blame for this state of affairs : the difficulties of 

 making the measurements and the lack of suitable apparatus have 

 largely contributed to it. In what follows, I propose to draw atten- 

 tion to some existing apparatus and methods which can be applied to 

 this purpose. 



An investigation has been recently made by Professor P. E. Shaw* 

 on the amplitude of the movement of a telephone diaphragm by means 

 of an extremely sensitive micrometer which he has devised. I will 

 cite one result as showing how extremely small are the quantities 

 with which we have to deal in telephony. He finds that the move- 

 ment of the diaphragm corresponding to a just comfortably loud 

 impulsive sound is only one twenty-thousandth part of a millimetre, 

 and that something less than one-fiftieth of this is still just audible. 



The diaphragm has a frequency of vibration of its own which in 

 ordinary receivers may be about 500 complete vibrations per second ; 

 it will therefore tend to reinforce (due to resonance) notes having the 

 same frequency as itself, i.e. about the octave above middle C. This 

 leads to the very unpleasant accentuating of certain notes, when 

 music is transmitted telephonically. It seems as if this might be 

 overcome by applying some form of damping to the diaphragm or by 

 making its frequency of vibration very much higher. 



To test the electrical part of the apparatus, we require some means 

 of measuring small alternating currents of fairly high frequency and 

 also some method of producing these currents. At first sight it 

 would seem comparatively easy to construct an alternator to produce 

 these currents, as the highest frequency does not exceed about 2000 

 periods per second, and alternators have been constructed to give very 

 much higher frequencies. t The real difficulty is to obtain a machine 

 which will give a strictly sinusoidal current under all conditions. 

 This is necessary to enable the experimental results to be easily com- 

 pared with theory. 



There are other methods of producing high-frequency currents, 

 such as : (1) the Humming Telephone % ; (2) the Musical Arc § ; (8) 

 the Musical Vacuum Tube, which is produced by shunting a vacuum 

 tube, supplied with high voltage direct current, with a condenser and 

 self-induction in series in a similar way to the musical arc ; (4) the 

 vibrating bar of Mr. Campbell || which works in a manner analogous 

 to the electrically maintained tuning fork, except that the contact 

 is replaced by a small microphone. This latter apparatus gives a 

 very constant frequency and current. 



Although electromagnetic instruments such as dynamometers liave 

 been constructed sufficiently sensitive to measure telephonic currents, 



* Proc, Roy. Soc, Ixxvi. pp. 350-366. 



t Proc. Phys. Soc, xix., p. 299, 1905 ; also Phil. Mag. 



j F. Gill, Journ. Inst. Elec. Eng., xxxi. p. 388, 1902. 



§ Journ. Inst. Eloc. Eng., xxx., 1901 ; and Proc. Roy. Inst., Feb. 1902. 



11 Proc. Phys. Soc, xix., p. 171, 1904. 



