650 The Striking Point of Pianoforte Strings. 



From mirror to film was 164*0 cm. 



The magnification is therefore 2 x 161'0-f-0"6 = 517. 



In No. 9 the film was only 77 cm. from the mirror but 

 the blow was somewhat greater. 



On examining the photographs it is at once apparent that 

 there are two distinct vibrations recorded on each film, one 

 vibration being about five times the pitch of the other. 



The vibrations of the string are transmitted through the 

 bridge (shown immediately above the cross-bar carrying the 

 mirror in fig. 1) to the sounding-board, forcing the latter to 

 vibrate with the same period as that of the string. Now the 

 sounding-board K r is fixed only at its two ends, and together 

 with the whole length of the string it must have a natural 

 period of its own. Any blow upon the string or bridge will 

 set the sounding-board vibrating with its natural period, and 

 this vibration together with the forced vibration due to the 

 string appear upon all the films. 



A regular variation in the intensity of the light will be 

 noticed. This is due to the alternating current passing- 

 through the arc. The pitch of this oscillation was about 

 50 per second, but is stated to vary somewhat. The pitch of 

 the string being 261 it is at once apparent that the longer 

 wave is the one due to the natural period of the sounding- 

 board. 



The straight white line shows the spot of light as the weight 

 is falling, and the effect of the blow on the string is to drive 

 the spot of light in many cases off the film. The film being 

 on a cylinder, when exposed the wave passes immediately 

 from the right-hand edge of the film to the left-hand edge. 



Taking an average of the respective amplitudes of the 

 vibrations the results have been plotted in the form of a 

 curve in the accompanying diagram. The curve A gives 

 the amplitude of the forced vibration of the sounding-board 

 produced by the vibration of the string. The curve B 

 gives the amplitude of the natural vibration of the sounding- 

 board. It is the aim of the pianoforte maker to obtain the 

 maximum effect from the string and the minimum effect from 

 the natural period of the sounding-board. It will be seen 

 that this point is reached when the strino- is struck by the 

 hammer at \ of its length from the fixed bridge. 



The pianoforte maker has reached the same result by 

 empirical methods. For this particular string, measurement 

 of the striking-point on two English pianos gave ~g and g^ 



respectively as the point chosen, and on a German piano g^. 

 The natural vibration of the sounding-board falls off very 



