18 Prof. Hughes on the Physical 



up if the pressure be removed, as sand is on a vibrating glass. 

 The fchrowing-up of this upper piece B when pressure -is re- 

 moved proves that a blow, pressure, or upheaval of the lower 

 portion takes place : that this takes place there cannot be any 

 doubt, as the surface, considered alone (having no depth), 

 could not bodily quit its mass. In fact, there must have been 

 a movement to a certain depth ; and I am inclined to believe, 

 from numerous experiments, that the whole block increases 

 and diminishes in size at all points, in the centre as well as 

 the surface, exactly in accordance with the form of the sono- 

 rous wave. Confining our attention, however, to points on 

 A and B, how can this increased molecular size or form pro- 

 duce a change in the electrical waves ? This may happen in 

 two ways : — -first, by increased pressure on the upper surface, 

 due to its enlargement ; or, second, the molecules themselves, 

 finding a certain resistance opposed to their upward move- 

 ment, spread themselves, making innumerable fresh points of 

 contact. Thus an undulatory current would appear to be pro- 

 duced by infinite change in the number of fresh contacts. 

 I am inclined to believe that both actions occur : but the latter 

 seems to me the true explanation ; for if the first were alone 

 true, we should have a far greater effect from metal powder, 

 carbon, or some elastic conductor, such as metallized silk, than 

 from gold or other hard unoxidizable matter ; but as the best 

 results as regards the human voice were obtained from two sur- 

 faces of solid gold, I am inclined to view with more favour 

 the idea that an infinite variety of fresh contacts brought into 

 play by the molecular pressure affords the true explanation. 

 It has the advantage of being supported by the numerous 

 forms of microphone I have constructed, in all of which I can 

 fully trace the effect. 



1 have been very much struck by the great mechanical 

 force exerted by this uprising of the molecules under sonorous 

 vibrations. With vibrations from a musical box 2 feet in 

 length, I found .that one ounce of lead was not sufficient on a 

 surface of contact 1 centim. square to maintain constant con- 

 tact ; and it was only by removing the musical box to a dis- 

 tance of several feet that I was enabled to preserve continuity 

 of current with a moderate pressure. I have spoken to forty 

 microphones at once ; and they all seemed to respond with 

 equal force. Of course there must be a loss of energy in the 

 conversion of molecular vibrations into electrical waves ; but 

 it is so small that I have never been able to measure it with 

 the simple appliances at my disposal. I have examined every 

 portion of my room — wood, stone, metal, in fact all parts — and 

 even a piece of india-rubber : all were in molecular move- 



