366 Prof. Hughes on the Action of Sonorous Vibrations [May 9 t 



in its present form, consists simply of a lozenge-shaped piece of gas 

 carbon, one inch long, quarter inch wide at its centre, and one-eighth 

 of an inch in thickness. The lower pointed end rests as a pivot upon 

 a small block of similar carbon ; the upper end, being made round, 

 plays free in a hole in a small carbon-block, similar to that at the 

 lower end. The lozenge stands vertically upon its lower support. 

 The whole of the gas carbon is tempered in mercury, in the way 

 previously described, though this is not absolutely necessary. The 

 form of the lozenge-shaped carbon is not of importance, provided the 

 weight of this upright contact piece is only just sufficient to make a 

 feeble contact by its own weight. Carbon is used in preference to any 

 other material, as its surface does not oxidise. A platinum surface in 

 a finely-divided state is equal, if not superior, to the mercurised carbon, 

 but more difficult and costly to construct. I have also made very 

 sensitive ones entirely of iron. 



The best form and materials for this instrument, however, have not 

 yet been fully experimented on. Still, in its present shape, it is 

 capable of detecting very faint sounds made in its presence. If a pin, 

 for instance, be laid upon or taken off a table, a distinct sound is 

 emitted, or, if a fly be confined under a table-glass, we can hear the 

 fly walking, with a peculiar tramp of its own. The beating of a pulse, 

 the tick of a watch, the tramp of a fly, can thus be heard at least 

 a hundred miles distant from the source of sound. In fact, when 

 further developed by study, we may fairly look for it to do for us, 

 with regard to faint sounds, what the microscope does with matter too 

 small for human vision. 



It is quite evident that these effects are due to a difference of pres- 

 sure at the different points of contact, and that they are dependent for 

 the perfection of action upon the number of these points of contact. 

 Moreover, they are not dependent upon any apparent difference in the 

 bodies in contact, but the same body in a state of minute subdivision 

 is^ equally effective. Electrical resistance is a function of the mass of 

 the conductor, but sonorous conduction is a function of the molecules 

 of matter. How is it therefore that a sonorous wave can so affect the 

 mass of a conductor as to influence its electrical resistance ? If we 

 assume a line of molecules, we know that a sonorous wave is accom- 

 panied by alternate compressions and rarefactions. If we isolate the 

 part under compression from the part under dilatation we vary the 

 dimensions of the mass, and we alter its electrical resistance. In any 

 homogeneous conductor of finite dimensions the effect of the one will 

 exactly compensate for the effect of the other, and we get no varia- 

 tion of current, but if we break up this homogeneous conductor into a 

 series of minute subdivisions without actually breaking their electrical 

 continuity we destroy this neutralizing influence, and we render 

 evident the effect of sonorous vibrations in varying the dimensions of 



