connected with the Telephone. 283 



in acoustics, that wherever there is sound there is always vi- 

 bration. Sound and vibration are concomitant and inseparable 

 phenomena. The air cannot produce sound unless it is thrown 

 into vibration ; and the air itself cannot be thrown into vibra- 

 tion unless the mass of matter in contact with it vibrates also. 

 The amplitudes of vibration of the particles of the air themselves „ 

 have never been measured, though the length of a sound-wave 

 (a very different quantity) is accurately known. Lord Rayleigh 

 has shown that an amplitude of only , n n * n n n of a centimetre 



1 / 10000000 



is sufficient to produce sonorous vibrations. But though the 

 amplitude of the vibrations be so small they are rapid. Now 

 this rate of motion is sufficient to bend in the same ratio the 

 lines of force cutting a b, and thereby to produce currents of 

 electricity in the ring a b whose number depends on the num- 

 ber of vibrations, and whose form and intensity depend on the 

 rate and amplitude of motion of the disk e. These currents 

 are alternate, and so rapid that no known instrument but the 

 telephone indicates them ; but they are readily shown by a 

 Thomson's reflecting galvanometer when the disk is gently and 

 slowly pressed in by the finger, — in one direction when the 

 disk is pressed in, in the other direction when the disk is allowed 

 to fly back. 



I have failed hitherto to make even an approximate mea- 

 surement of their minuteness. We have no known standard 

 to compare them with : we can only trust to the ear ; and that 

 instrument is not only deceptive but variable. They are cer- 

 tainly less than -, nn } nnn of an ordinary working current. 

 J _. 1000000 J n 



Mr. E. S. Brough, of the Indian Government Telegraph De- 

 partment, has calculated that the strongest current with which a 

 telephone is at any moment worked does not exceed 100 ooooooo 

 of the C. G. S. unit, or weber ; and Professor Pierce, of Boston, 

 found that similar effects are produced with an electromotive 

 force of less than * of a volt or Danieli's cell. Thus Ave 

 have a source of electricity competent to produce currents of 

 microscopic strength, which vary in form, duration, and inten- 

 sity with the motion of the body producing them. 



II. The Telephone as a Detector. 

 Let n s, fig. 2, be a core of soft iron surrounded 

 by a closed conductor a!V ', through which currents 

 flow. Now this core will become magnetized with 

 an intensity dependent solely upon the intensity ^ 

 of the current ; and the intensity of magnetism at 

 any moment will be a function of the intensity 

 of the current at that moment; so that if the 

 current increase and decrease with a given ratio 



