August 14, 1879] 



NATURE 



361 



pecuniary results, though more accordant Avith ordinary 

 experience. Let the reader of these eight chapters on 

 " Farming for Pleasure and Profit" omit everything ex- 

 cept what is given on the personal authority of the author, 

 and he will gain a number of useful hints showing how 

 to economise the vegetable food raised on a small farm, 

 and to make amateur agriculture in some ways less finan- 

 cially disastrous than is usually the case. But we shall 

 not find a complete system of practice here ; nor do we 

 discover any hints, however remote, of the chemical com- 

 position and physiological functions of food ; and we 

 ;ook, too, in vain for any recognition of recent advances 

 in our scientific knowledge as to methods of manuring 

 and cropping. 



LETTERS TO THE EDITOR 



[7^ht Editor does not hold himself responsible for opinions expressed 

 by his c»rresfondents. Neither can he undertake to return, or 

 to correspond with the writers of, rejected manuscripts. No 

 notice is taken tf antnymnis communications. 



[ Tki Editor urgently requests ctrrespondents to keep their letters as 

 shtrt as possible. J he pressure on his space is so great that it 

 is impossible tthenvise to ensure the appearance even of cofn- 

 munications containing interesting and novel facts.^ 



Theory and Laws of the Microphone 



Two hypotheses have been projected to explain the action of 

 the microphone. One ,is molecular exclusively, and supposes 

 that the molecules of certain conducting bodies contract and 

 dilate under sonorous vibrations. Changes of density correspond 

 with an increase or diminution of the resistance of the circuit. 

 This hypothesis renders the phenomena analogous to those which 

 selenium presents under the influence of light and radiant heat. 

 The other explanation, partly mechanical and partly molecular, 

 is the result of a discovery made some time since by M. du 

 Moncel, according to whom the increase or diminution in resist- 

 ance is due to changes of pressure at the points of electric 

 contact. These changes of pressure are effected by the vibration 

 of the air ; hence the cause and the effect are similar. 



As the result of numerous experiments, I shall endeavour to 

 prove that, while one of these theories is altogether erroneous, 

 the other is only superficially true. 



1. If the piece of charcoal is fixed with wax without any 

 pressure, the microphone remains silent under the strongest 

 sonorous impulse?, which would be impossible were the move- 

 ment molecular. 



2. The microphone may be inclosed in a vacuous chamber 

 without altering the result ; in this case waves of air can have 

 no effect upon the density of the charcoal. 



3. It is impossible to construct a microphone from one solid 

 piece of charcoal, presenting stable contact, such as would not 

 interfere with molecular action, but which prevents the sonorous 

 waves from affecting the cturrents which traverse the carbon. 



These affi rd sufficient reasons for rejecting any simply mole- 

 cular theory. 



Now against the second theory. 



1. Lateral pressure on a compact electric conductor excites no 

 microphonic action. 



2. Longitudinal pressure within certain limits on the charcoal 

 does not injure the apparatus. 



3. An apparatus can be made to yield microphonic effects 

 where there is no alteration of pressure. Pressure, therefore, 

 is no essential cause of microphonic sounds, though it may be an 

 accidental one. 



In all microphones where contact is made at one point only, 

 the current is interrupted whenever this point of contact is 

 broken ; a musical sound is heard when the two points are in 

 vibration. This microphone, like Reis's telephone, can only 

 transmit musical sounds. I have obtained the best results from 

 a steel point and a membrane of a stretched bladder. A strip 

 of tin-foil is gummed to the membrane to insure electric contact. 

 With a single small cell of bichromate of potash a song can be 

 heard through a whole room. 



To transmit articulate sound it is necessary that the number of 

 points of contact, the difference in this number during action, 

 and the resultant changes of resistance, should be greater. The 

 interruption of the current is then only partial; it becomes 



" uiidulatory." . To this dcicription belong the principal micro, 

 phone of Mr. Hughes, Edison's carbon telephone, the trans- 

 mitting telephone of two graphite pencils of MM. Pollard and 

 Gamier, Ilelleren, &c. 



A convenient form of microphone, which transmits words, 

 music, the noise of a watch, &c., has the membrane of india- 

 rubber stretched tight by a thin strip of tin-foil which unites 

 the carbon underneath with the screw. The vibrations of the 

 membrane throw a greater or less number of points into con- 

 tact ; all the shades of expression in the voice may be transmitted, 

 owing to the rapidity of these small changes. It is the changes 

 in the points of contact which here play the chief part, and 

 there is little doubt that here we have the quaUty as well as the 

 intensity of a sound reproduced. 



This explains many of the microphonic actions, hut not all. 

 Here is one case : — 



If the microphone is formed of two cylindrical pieces of char- 

 coal, the points of contact cannot be made to vary by pressure, 

 supposing the cylindrical shape to be perfect. The action he e 

 is due to the distance the current has to traverse the bad c 1 n- 

 ductor ; for the membrane to which these charcoal points are 

 attached approaches or recedes from them when vibrating. The 

 liquid telephone transmitters of Bell, Gray, and Salet rest upon 

 the same principle where a change of resistance in the circuit is 

 due to the varying depth of liquid traversed by the current. This, 

 equally with the theory of the points of contact, explains the 

 microphone of M. Kighi, where a metallic disk is plunged into 

 a powder of lead and silver mixed. 



The next class of instruments consists of those where the 

 current is created, and varies under the influence of microphonic 

 electrodes (by this I mean the opposite parts of a microphone, 

 whether in direct or indirect contact). Such are microphonic 

 batteries. Each battery can act as a microphone if one of its 

 poles is movable. Two ends of iron wire, dipped in ordinary, 

 water, and brought together, give signs of microphonic action.' 

 One pole is attached to the vibrating membrane, and dips at its 

 extremity into the liquid, while the other pole remains there con*; 

 stantly. The current only passes when the movable pole is in 

 the liquid. On singing into the tube the vibrations of the mem- 

 brane cause the pole, which is also a microphonic electrode, to 

 dip into the liquid, setting up chemical action as many times a 

 second as there are vibrations in the note sung. If the pole 

 touches the liquid constantly, the current is constant, but varies 

 in intensity for four reasons : the different number of points ex-, 

 posed to electrolytic action ; the different number of points of 

 electric contact ; the different number of the points of resistance 

 of the liquid ; and the different number of the points of approach' 

 of one pole to the other ; all these are due to the moveinent of 

 one of the microphonic electrodes. 



There is yet another class of microphones. In all the instru- 

 ments hitherto constructed, the direction of the current remains 

 the same, but it is possible to make it change, thus introducing 

 another difference in the manner of its action. 



In all the possible forms of microphone, the chief causes of the 

 action are : — a mechanical movement of its parts, a change in 

 the points of conductivity, a change of resistance ; these three 

 essentials result from one another. The expression " points of 

 conductivity " includes not only the points of contact, but also 

 the route taken by the current. 



The next point of consideration is the so-called increase of 

 sound by a microphone, but this is not the case. All sounds' 

 are weakened by the microphone, and are transmitted only when", 

 the source of sound is in direct contact with the microphone or' 

 its stand. The microphone is less an instrument for transmitting • 

 sound than for transforming mechanical movement into sound. 

 The intensity of a sound is, therefore, directly proportional to 

 the energy of the mechanical movement accompanying the 

 sonorous waves, or as a necessary consequence to the changes of 

 resistance in the microphone. The distinctness of articulate 

 sounds, transmitted by the microphone, is in inverse proportion 

 to their intensity ; for a loud sound tends to interrupt completely 

 the current, and thus to prevent the transmission of articulate 

 sounds. This is the chief hindrance to increasing, at pleasure, 

 the loudness of the sound. The loudness of the sound is also 

 dependent on the strength of the current. 



Other experiments prove that the rapidity of movement of the 

 parts of a microphone also affects very considerably the resultant 

 sound, as well with a strong, as with a feeble current. Changes 

 in resistance and in current strength are not sufficient, unless 

 made rapidly, to excite microphonic action. 



