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ON THE QUALITY OF MUSICAL SOUNDS. 7T
tuning-forks, differing in pitch by an exact octave, and mount them on resonating boxes of the proper dimensions; set the lower one vibrating, by bowing it with a violin or double bass bow, and note the dull yet gentle effect of the simple tone produced. Now bow both the forks rapidly one after the other; the two simple tones will soon coalesce, and will sound to the ear as one tone, of the pitch of the lower one, but of much brighter quality than before. The effect of the higher fork, that is, of the second partial, will be strikingly seen, by damping it after both have been vibrating a second or two; the return to the original dull simple tone is very marked. This experiment may be varied very greatly, by the aid of four or five forks tuned to the first four or five partials. The following eight forks form a very serviceable series for these experiments.
Of course, the effect produced by these forks is only an approxi­mation to the effect produced by the real partials, for, in the first place, the forks cannot very easily be all excited at the same instant; and again, their intensities can only be regulated in a very rough way. In an arrangement devised by Helmholtz for investigating the vowel sounds, these two difficulties were removed by the use of electro-magnets for exciting the forks, and by employing resonators at different distances, and with moveable openings, to regulate their intensities.
The way in which a tuning-fork is excited by an electro-magnet will be understood by a reference to fig. 39. Let A and B be the poles of an electro-magnet, and C and D the ends of the prongs of a tuning-fork between them. If now a current be sent through the electro-magnet, the poles A and B will attract C and D. Now if the current be stopped, A and B will cease to attract the prongs, which will therefore move towards one another again in consequence of their elasticity. Let the current again pass, and 0 and D will again be attracted. If we can thus alternately pass and stop the current, every time the prongs move forward and backward, the