314 



SCIENCE. 



UPON A MODIFICATION OF WHEATSTONE'S 

 MICROPHONE. AND ITS ADAPTABILITY 

 TO RADIOPHONIC RESEARCHES.* 



By Alex. Graham Bell. 



In August, 1880, I directed attention to the fact that 

 thin discs or diaphragms of various materials become 

 sonorous when exposed to the action of an intermittent 

 beam of sunlight, and I stated my belief that the sounds 

 were due to molecular disturbances produced in the sub- 

 stance composing the diaphragm. 1 Shortly afterward, 

 Lord Raleigh undertook a mathematical investigation of 

 the subject, and came to the conclusion that the audible 

 effects were caused by the bending of the plates under 

 unequal heating. 2 This explanation has recently been 

 called in question by Mr. Preece, 3 who has expressed the 

 opinion that, although vibrations may be produced in the 

 discs by the intermittent beam, such vibrations are not 

 the cause of the sonorous effects observed. According 

 to him the aerial disturbances that produce the sound 

 arise spontaneously in the air itself by sudden expansion 

 due to heat communicated from the diaphragm — every 

 increase of heat giving rise to a fresh pulse of air. Mr. 

 Preece was led to discard the theoretical explanation of 

 Lord Raleigh on account of the failure of experiments 

 undertaken to test the theory. 



He was thus forced, by the supposed insufficiency of 

 the explanation, to seek in some other direction the cause 

 of the phenomenon observed, and, as a consequence, he 

 adopted the ingenious hypothesis alluded to above. 

 But the experiments which had proved unsuccessful in 

 the hands of Mr. Preece, were perfectly successful when 

 repeated in America under better conditions of experi- 

 ment, and the supposed necessity for another hypothesis 

 at once vanished. I have shown in a recent paper read 





A 



C 



B 









<l . jJ 



m ' 



















Fig. 1. 



before the National Academy of Science 4 that audible 

 sounds result from the expansion and contraction of the 

 material exposed to the beam, and that a real to and fro 

 vibration ot the diaphragm occurs, capable of producing 

 sonorous effects. It has occurred to me that Mr. 

 Preece's failure to detect with a delicate microphone the 

 sonorous vibrations, that were so easily obseerved in our 

 experiments, might be explained upon the supposition that 

 he had employed the ordinary form of Hughes' micro- 

 phone shown in Fig. 1, and that the vibrating area was 



* A paper read before the Philosophical Society of Washington, D. C. 

 June 11, 188I. 



1 American Association for Advancement of Science, August 2~, 1880. 



3 Nature, vol. xxiii., p. 274. 



3 Royal Society, March 10, 1881. 



1 April m, 1881 



confined to the central portion of the disc. Under such 

 circumstances it might easily happen that both the sup- 

 ports, a b, of the microphone might touch portions of the 

 diaphragm which were practically at rest. It would, of 

 course, be interesting to ascertain whether any such 

 localization of the vibration as that supposed really 

 occurred, and I have great pleasure in showing to you 

 tc-night the apparatus by means of which this point has 

 been investigated. 



Fig. a. 



The instrument is a modification of the form of micro- 

 phone devised in 1827 by the late Sir Charles Wheat- 

 stone, and it consists essentially of a stiff wire, A, one 

 end of which is rigidly attached to the centre of a metallic 

 diaphragm, B. In Wheatstone's original arrangement, 

 the diaphragm was placed directly against the ear, and 

 the free extremity of the wire was rested against some 

 sounding body — like a watch. In the present arrange- 

 ment, the diaphragm is clamped at the circumference like 

 a telephone-diaphragm, and the sounds are conveyed to 

 the ear through a rubber hearing-tube, c. The wire 

 passes through the perforated handle, D, and is exposed 

 only at the extremity. When the point A w r as rested 

 against the centre of a diaphragm upon which was 

 focussed an intermittent beam of sunlight, a clear, musi- 

 cal tone was perceived by applying the ear to the hear- 

 ing-tube c. The surface of the diaphragm was then 

 explored with the point of the microphone, and sounds 

 were obtained in all parts of the illuminated area and 

 in the corresponding area on the other side of the dia- 

 phragm. Outside of this area on both sides of the dia- 

 phragm, the sounds became weaker and weaker until, at 

 a certain distance from the centre, they could no longer 

 be perceived. 



At the points where we would naturally place the sup- 

 ports of a Hughes' microphone (see Fig. 1) no sound 

 was observed. We were also unable to detect any 

 audible effects when the point of the microphone was 

 rested against the support to which the diaphragm was 

 attached. The negative results obtained in Europe by 

 Mr. Preece may, therefore, be reconciled with the positive 

 results obtained in America by Mr. Tainter and myself, 

 A still more curious demonstration of localization of 



