SCIENCE. 



251 



Fig. 9. 



experimental purposes is shown in Fig. 9. When an 

 intermittent current is passed through the lamp-black, 

 (A) or when an intermittent beam of sunlight falls upon 

 it through the glass plate (B) a loud musical tone can be 

 heard by applying the ear to the hearing-tube (C). 

 When the light and the electrical current act simultan- 

 eously, two musical tones are perceived, which produce 

 beats when nearly of the same pitch. By proper arrange- 

 ments a complete interference of sound can undoubtedly 

 be produced. 



UPON THE MEASUREMENT OF THE SONOROUS EFFECTS 

 PRODUCED BY DIFFERENT SUBSTANCES. 



We have observed that different substances produce 

 sounds of very different intensities under similar circum- 

 stances of experiment, and it has appeared to us that 

 very valuable ififormation might be obtained if we could 

 measure the audible effects produced. For this purpose 

 we have constructed several different forms of apparatus 

 for studying the effects, but as our researches are not yet 

 complete, I shall confine myself to a simple description 

 of some of the forms of apparatus we have devised. 



When a beam of light is brought to a focus by means 

 of a lens, the beam diverging from the focal point be- 

 comes weaker as the distance increases in a calculable 

 degree. Hence, if we can determine the distances from 

 the focal point at which two different substances emit 

 sounds ot equal intensity, we can calculate their relative 

 sonorous powers. 



Preliminary experiments were made by Mr. Tainter 

 during my absence in Europe to ascertain the distance 

 from the focal point of a lens at which the sound pro- 

 duced by a substance became inaudible. A few of the 

 results obtained will show the enormous differences ex- 

 isting between different substances in this respect. 



DISTANCE FROM FOCAL POINT OF LENS AT WHICH 

 SOUNDS BECOME INAUDIBLE WITH DIFFERENT 

 SUBSTANCES. 



Zinc diaphragm (polished) 1.51 m 



Hard rubber diaphragm 1.90 " 



Tin-foil " 2.00 " 



Telephone " (Japanned iron) 2.15 " 



Zinc " (unpolished) 2.15 " 



White silk, (In receiver shown in Fig. 1.) 3.10 " 



White worsted, 

 Yellow worsted, 

 Yellow silk, 

 White cotton wool, 

 Green silk, 



4.01 

 4.06 

 4-13 

 4.38 

 4.52 



Blue worsted, " " " " 4.69 " 



Purple silk, " " " " 4.82 " 



Brown silk, " " " " 5.02 " 



Black silk, " " " " 5.21 " 



Red Silk, " " " " 5.24 " 



Black worsted, " " " " 6.50 " 

 Lamp-black. In receiver the limit of audibility 

 could not be determined on account of want of 

 space. Sound perfectly audible at a distance 



ot r 10.00 " 



Mr. Tainter was convinced from these experiments 

 that this field of research promised valuable results, and 

 he at once devised an apparatus for studying the effects, 

 which he described to me upon my return from Europe. 

 The apparatus has since been constructed and I take 

 great pleasure in showing it to you to-day. 



(1.) A beam of light is received by two similar lenses 

 (A B, Fig. 10*), which bring the light to a focus on either 

 side of the interrupting disk (C). The two substances, 

 whose sonorous powers are to be compared, are placed 

 in the receiving vessels (D E) — so arranged as to ex- 

 pose equal surfaces to the action of the beam— which 

 communicate by flexible tubes (F G) of equal length, 

 with the common hearing-tube (H). The receivers 

 (D E) are placed upon slides, which can be moved along 

 the graduated supports (I K). The beams of light pass- 

 ing through the interrupting disk (C), are alternately cut 

 off by the swinging of a pendulum (L). Thus a musical 

 tone is produced alternately from the substance in D 

 and from that in E. One of the receivers is kept at a 

 constant point upon its scale, and the other receiver is 

 moved towards or from the focus of its beam until the 

 ear decides that the sounds produced from D and E 

 are of equal intensity. The relative positions of the re- 

 ceivers are then noted. 



(2.) Another method of investigation is based upon 

 the production of an interference of sound, and the 

 apparatus employed is shown in Fig. 11.* The inter- 

 rupter consists of a tuning-fork (A), which is kept in 

 continuous vibration by means of an electro-magnet (B). 



A powerful beam of light is brought to a focus be- 

 tween the prongs of the tuning-fork (A), and the passage 

 of the beam is more or less obstructed by the vibration 

 the opaque screens (C D) carried by the prongs of the 

 fork. 



As the tuning-fork (A) produces a sound by its own 

 vibration, it is placed at a sufficient distance away to be 

 inaudible through the air, and a system of lenses is em- 

 ployed for the purpose of bringing the undulating beam 

 of light to the receiving lens (E) with as little loss as 

 possible. The two receivers (F G) are attached to slides 

 (H I) which move upon opposite sides of the axis of the 

 beam, and the receivers are connected by flexible tubes 

 of unequal length (K L) communicating with the com- 

 mon hearing-tube (M). 



The length of the tube (K) is such that the sonorous 

 vibrations from the receivers (F G) reach the common 

 hearing-tube (M) in opposite phases. Under these cir- 

 cumstances silence is produced when the vibrations in 

 the receivers (F G) are of equal intensity. When the 

 intensities are unequal, a residual effect is perceived. 

 In operating the instrument the position of the receiver 

 (G) remains constant, and the receiver (F) is moved to 

 or from the focus of the beam until complete silence is 

 produced. The relative positions of the two receivers 

 are then noted. 



(3). Another mode is as follows : The loudness of a 

 musical tone produced by the action of light is compared 

 with the loudness of a tone of similar pitch produced by 

 electrical means. A rheostat introduced into the circuit 

 enables us to measure the amount of resistance required 

 to render the electrical sound equal in intensity to the 

 other. 



* See pages 248 and 249 for illustrations. 



