496 



NA rURE 



[March 24, 1881 



ON THE CONVERSION OF RADIANT ENERGY 



INTO SONOROUS VIBRATIONS^ 

 lYfESSRS. GRAHAM BELL and Sumner Tainter (American 

 ■'•''•'- Association for the Advancement of Science, Boston, 

 August 27, 18S0) have shown that under certain conditions 

 intense rays of Ught, if allowed to fall with periodic intermit. 

 tence upon thin disks of almost every hard substance, will set 

 up disturbances in those disks corresponding to this periodicity 

 which result in sonorous vibrations. Mr. Bell (jfoiirnal of the 

 Society of Telegraph Engineers, December 8, i88o) has subse- 

 quently shown that such effects are not confined to hard sub- 

 stances, but that they can be produced by matter in a liquid 

 or gaseous form. 



These discoveries have elicited a considerable amount of in- 

 terest, and have led to the inquiry whether the sonorous effects 

 are due, as the discoverers themselves surmised, to lights or as 

 the president of the Royal Society, Prof. Tyndall, and others 

 have suggested, to radiant heat. 



Messrs. Bell and Tainter have partially answered this question 

 by showing that the disturbances are not necessarily due to light, 

 for they found that sheets of hard rubber or ebonite — a substance 

 opaque to light — do not entirely cut off the sounds, but allow 

 certain rays to pass through, which continue the effect. M. 

 Mercadier, who has studied the subject with great care [Comftes 

 reiidiis, December 6, l8So), lias shown that the effects are con- 

 fined to the red and ultra-red rays. Moreover Mr. Bell has 

 shown that gases, such as sulphuric ether, which Prof. Tyndall 

 has proved to be highly absorbent of heat rays, while they are 

 transparent to light rays, are remarkably sensitive to this inter- 

 mittent action. Dr. Tyndall has more recently read a paper 

 before the Society (Proc, Roy. Soc, January 3, 1881) proving 

 that these sonorous effects are a function of all gases and vapours 

 absorbing radiant heat, and that the intensity of the sounds pro- 

 duced is a measure of this absorption. 



The negative proof of Messrs. Bell and Tainter can be ren- 

 dered positive if it can be shown that ebonite is diatherntatioia. 



By very careful experiments made upon the diathermancy of 

 different materials, ebonite proved to be as diathermanous as 

 rock-salt. 



It is therefore clear that the sonorous vibrations of Bell and 

 Tainter are the result of disturbances produced by some thermic 

 action rather than by any luminous effect. 



Now the questions arise. Is this thermic action expansion and 

 contraction of the mass due to its absorption of heat? Or is it 

 a disturbance due to molecular pressure similar to that which 

 produces the rotation of the radiometer ? Or is it due to some 

 other cause ? 



The argument against the first assumption when applied to 

 hard disks is that time is a material element in such actions, and 

 that the rate of cooling of warmed diaphragms is too slow to 

 admit of such effects. Lord Rayleigh (Nature, January 20, 

 1881) has questioned the validity of this argument, and has 

 shown that if the radiating power of the body experimented on 

 were sufticiently high a slow rate of cooling would be favourable , 

 to rapid fluctuations of temperature. It became desirable to i 

 test this point experimentally. Very delicate apparatus was j 

 constructed for the purj ose. 



Heat from various sources and at various distances, from bright ' 

 lime-light to dull heat from hot metallic surfaces, was allowed 

 to fall through rotating vanes intermittently on different bodies ; 

 but notwithstanding every precaution, and the many materials 

 u-ed, not more than six interruptions per second could be pro- , 

 duced, although I he system was beautifully sensitive to the , 

 smallest changes of temperature. I 



It was evident from these experiments that the sonorous 

 effects of hard disks could not be explained by the change of 

 volume due to the impact of heat rays and their absorption by 

 the mass of the disk. 



Is the action then due to molecular pressure similar to that 

 which produces the rotation of the radiometer? 



It is quite true that the radiometer effect is one visible only in 

 very high exhaustions, but Mr. Crookes (Phil. Trans., 1878, 

 Part i., §220) detected "the existence of molecular pressure 

 when radiation falls on a black surface in air of normal 

 density." 



Whenever radiant energy falls on an absorbent surface in air, 

 such as a disk of blackened wood, its wave-length is degraded 



^ Abstract of a paper by Mr. William Henry Preece, read at the Royal 



or lowered, and it is converted in thermometric heat. The 

 molecules of air striking this warmed surface acquire heat, and 

 move away from it with increased velocity, and as action and 

 reaction are always alike in moving away, they give the body a 

 " kick." Since there is no such action on the other side of the 

 disk, there is a difference of pressure between the two sides 

 which gives it a tendency to move away from the source of 

 energy. The effect is very much smaller in air at ordinary 

 pressures than in air at a very low vacuum, because in the 

 former case the mean free path of the molecules is very small, 

 and the rebounding molecules help to keep back the more 

 slowly approaching molecules. Nevertheless, molecular pressure 

 is experienced, and if of sufficient magnitude and rapidity, it 

 ought to produce sonorous vibrations. It seemed probable that 

 the element of lime does not enter here so prominently as in 

 the previous case, for the radiometer effect is a mere surface 

 action of the disk, and not one affecting its mass. Hence it 

 was hoped that the retarding effects would be eliminated. If j 



the sonorous action, therefore, be due to a radiometer action, a ' 



difference of effect would be observed if the side of a disk 

 exposed to the source of energy be either blackened by lamp- 

 black or camphor carbon, or if it be polished or w hitened. 



An apparatus was constructed similar in principle to that 

 described by Messrs. Bell and Tainter. 



An ebonite disk well blackened on one side when exposed to 

 the intermittent rays was found to produce sounds, while a 

 similar ebonite disk equally well-whitened, gave slightly less 

 intense sounds. A zinc disk blackened gave weak sounds, while 

 a similar disk polished gave sounds much weaker. A mica disk 

 blackened gave scarcely any sounds at all, while a clean mica 

 disk gave slightly better sounds. 



These effects were produced many times and. on different 

 occasions, and they were so unsatisfactory as to throw doubts on 

 the accuracy of the radiometer explanation. They were not so 

 decided as theory led one to anticipate. The effects produced 

 by the zinc disk, though very weak, favoured the theory ; those 

 given by the mica disk completely refuted it ; while those given 

 by the ebonite disks were almost of a neutral character. 



The question occurred whether in Bell and Tainter's experi- 

 ments the disks vibrated at all. 



A delicate microphone was fi.xed in various ways on the case 

 holding the disks. Although the sounds emitted in the hearing- 

 tube were quite loud, scarcely any perceptible effect was detected 

 on the microphone. Had the disk sensibly vibrated, its vibra- 

 tions must have been taken up by the case. A microphone, 

 never fails to take up and magnify the minutest mechanical 

 disturbances. 



It was thus evident that the disk did not play a prime part in 

 this phenomenon, but that the result might be due wholly to an 

 expansion and contraction of the air contained in the air space 

 behind the disk. 



With anew clean case and an ebonite disk the sonorous effects 

 were feeble, but if a lens were placed close in front of the 

 ebonite disk, so as to make a second air space in front of the 

 disk, the sonorous effects were magnified considerably. 



The ebonite di^k was fitted with an extremely delicate micro- 

 phone, which in this case gave good indications upon the tele- 

 phone, but whether the vibrations were the results, of the 

 vibrations of the disk itself, or of the air in which the microphone 

 was placed, was doubtful. 



If the lens were removed and the disk left supported without 

 any air cavity, either behind or in front of it, no perceptible sound 

 could be obtained, proving that the effects were really due to the 

 vibrations of the confined air, and not to those of the disk. It 

 was therefore determined to dispense with the disk altogether. 

 The disk was therefore removed, the lens remaining ; the 

 sonorous effects were nil. 



Another case was taken under similar circumstances, i.e. with- 

 out the disk, but the effects were very loud— 60 ; in fact, the best 

 results which had yet been obtained. Now the only difference 

 between the one case or cup and the other was that the one was 

 blacked in the interior, and the other was not. 



Hence the former ca^e was again taken without the disk, and 

 though when clean it gave no effect, when its interior was blacked 

 by camphor smoke, it gave sounds as strong as the loudest effect 

 yet produced. It was thus evident that the sonorous effects were 

 materially assisted by coating the sides of the containing vessel 

 with a highly-absorbent substance, such as the carbon depo.sited 

 by burning camphor. It remained to be seen how far the lens 

 played a part in this phenomenon. 



