[ 652 ] 



LXIX. The Pianoforte Sounding- Board, 

 By G. H. Berry *. 



[Plate XII.] 



IN a recent number of this Magazine f it was shown that 

 a section of the sounding-board of a pianoforte had a 

 natural period of vibration of its own, independent of the 

 pitch of the strings upon it, and this natural vibration was 

 apparent in all the photographs there reproduced. 



With the particular section used the pitch of the sounding- 

 board was roughly 50 and that of the string 261. 



If the pitch of the sounding-board was the same as that 

 of the string, it seemed reasonable to expect the sound to be 

 considerably re-inforced, and the sounding-board to act as 

 the air-column acts, in the usual resonator fixed to the stem 

 of a suitable tuning-fork. 



As will be shown, this did not prove to be the case, or at 

 least a very important modification is necessary. 



Apparatus. 



The apparatus used was similar to that described in the 

 article mentioned, but several improvements have been made. 

 The photographic shutter was changed from 1/4 plate size to 

 1/1 plate to give a larger aperture. Instead of this shutter 

 being opened direct, the tube from the releasing bulb was 

 connected to the small shutter. The large shutter, giving 

 the actual exposure, was released by means of a small electro- 

 magnet. Two dry cells gave the necessary current and the 

 circuit was completed by the small shutter, at any desired 

 instant up to about 2 seconds, after the hammer had struck 

 the strings. 



To determine the speed of the film at the time of exposure, 

 a large tuning-fork was used as an interrupter. A wire from 

 one prong of the fork touched the surface of mercury in a 

 small cup, when the fork was at rest. When the fork was 

 vibrating it " made and broke " an electric circuit consisting 

 of the fork, mercury cup, two accumulators, the primary of 

 an induction-coil, and two contacts, one on either side of the 

 shutter. 



The two contacts pressed very lightly on the wings of the 

 shutter, and when the wings flew back, on the shutter being 

 released, the contacts came together and the current passed 

 through the induction-coil, etc., while the shutter was open, 



* Communicated by Prof. Edwin H. Barton, D.Sc, F.R.S.E. 

 t April 1910, p. 648. 



