On the Pianoforte Sounding-Board. 653 



the contacts being insulated again when the wings flew back 

 into position. The spark from the secondary of the coil 

 marked the film 3'5 cm. behind the exposure line. 



The frequency of the fork was determined by a strobo- 

 scope method. The mean of several results gave 21*50 

 vibrations a second. Thus the distance between the centres 

 of two time marks on the film represents ^ sec. 



The speed of the phonograph drum carrying the film 

 was kept as closely as possible the same lor all the exposures, 

 and was about 69 cm./sec. Thus the whole length of the film 

 passed the exposure line in 0*25 sec. 



The magnification on the films in every case is of the 

 order 500. 



In the course of the investigation several different sections 

 of sounding-board were used. They varied in length and 



Fig. 1. 



DETAILS OF SOUNDING BOARDS. 



SECTION C. 



PLANS. 



Of 



K 7 K 3 



SECTIONS 



CCMTl METRES 



thickness and therefore had different natural frequencies. A 

 drawing of these sections and the way in which they were 

 supported is shown in fig. 1. The wood used for the 

 sounding-board was that known commercially as " Swiss 

 Pine." The bar at the back of sounding-board was of Spruce 



