Charles Wheatstone to Faraday   23 March 1831¹

Dear Sir,

I have perused your paper and am quite satisfied that the inferences you draw from your experiments are correct². But I must differ from you in one thing; you state it as your opinion that it is of much less importance to establish the real cause of the phenomena than to show that Savart’s³ explanation is erroneous⁴. Now this is paying too much deference even to a highly merited reputation, and were this the sole utility of your experiments the end might have been obtained with much less time and ingenuity than you have bestowed upon the task. The fact that we do not hear these presumed co-existent sounds, is alone a sufficient evidence of their non-existence, nor can it be alleged in justification of the supposition that they are too feeble to be heard, for if they exist they are sufficiently intense to form acoustic figures; neither will the explanation avail by which the late Dr. Young attempted to account for the phenomena exhibited by liquids on vibrating elastic surfaces when I repeated my experiments to him, viz: that they might indicate the coexistence of vibrating motions so rapid as to exceed the limits of audibility; for the sounds assumed in Savart’s explanation are in general not more than two octaves above the fundamental sound. Another way by which Savart’s hypothesis can be satisfactorily disproved, is by an application of my optical means for decomposing vibratory motions with which you are acquainted⁵; this was the form of the experiment[.] I took a square plate of glass and at the middle of one of the edges I cemented a bead such as used in the Kaleidophone, and by applying a violin bow at the middle of another edge I caused the plate to vibrate in the mode of division with two diagonal quiescent lines; the light of a candle reflected from the bead formed a luminous line equal in length to the amplitude of the vibration which, on moving the plate rapidly in its own plane, was decomposed into a zig-zag line or but with out any trace of another zigzag line with more numerous bends and of lesser amplitude, which always occurs when in the vibrations of a string a higher sound coexists with the fundamental.

The real value of your experiments is that of assigning the true cause of phenomena, which was so little obvious as to have escaped the observation of such experienced philosophers and successful experimentalists as Chladni⁶, Oersted and Savart; and this discovery has a twofold importance, it will render the investigation of the residual phenomena of elastic surfaces less complicated, and it promises some further valuable information from the application of similar considerations to other phenomena with which they are intimately connected.

I remain | Dear Sir | Yours Truly | C. Wheatstone

March 23d | 31

Address: M. Faraday Esq | Royal Institution