HANDBOOK OF ACOUSTICS - online book
A complete view of Acoustical Science & its bearings on music, for musicians & music students.
|
ON INTERFERENCE. |
139 |
||
 |
|||
|
|
|||
|
Fig 70.
will be at rest, as indicated by the uniform shading. Precisely the same thing would occur, if A and B were three or any number of half wave-lengths apart.
Sir John Herschel made use of this principle in the construction of the apparatus shown in fig. 71. The tube of, which should be |
|||
|
|
|||
 |
|||
|
|
|||
|
Fig. 71. |
|||
|
longer than represented in the figure, divides into two at /, the one branch being carried round m, and the other round n. These two branches again unite at g, to form the tube g p. The U shaped portion n b, which slides air-tight by telescopic joints over the main tube a b, can be drawn out, as shown in the figure. When a vibrating fork is held at o, the sound waves produced, divide at/, and pass along the two branches, reuniting at g, before reaching the ear of the observer at p. Now if the U shaped portion is pushed home to a, the waves through both branches reach the ear together; but if it be gradually pulled out, a point is reached at which the sound disappears altogether. From what has been said above, it |
|||