A. G. Bell— Production of Sound by Radiant Energy. 479 
reproduction of articulate speech as well as for the production 
of musical sounds. 
We have also found that loud sounds are produced from 
lamp-black by passing through it an intermittent electrical 
current; and that it can be used as a telephonic receiver for 
the reproduction of articulate speech by electrical means. 
A convenient mode of arranging a lamp-black cell for ex- 
perimental purposes is shown in fig. 9. When an intermittent 
current is passed through the lamp-black (A,) or when-an in- 
termittent 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. hen the light and the electrical cur- 
rent act simultaneously, two musical tones are perceived, which 
produce beats when nearly of the same pitch. By proper ar- 
rangements 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 circumstances of ex- 
periment, and it has appeared to us that very valuable informa- 
tion might be obtained if we could measure the audible effects 
produced. For this purpose we have constructed several dif- 
ferent 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 becomes 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 of equal intensity, we can 
you to-day. 
