100 



Mr. Sudhansukumar Banerji on Aerial 



The mirror then comes back to the pointer, which brings it to 

 rest. The deflexion of the spot o£ light on the distant screen 

 gives a measure of the kick given to the mirror. It is found 

 that the apparatus is extremely sensitive, very faint sounds 

 being sufficient to produce deflexions which can be read off 

 by eye nearly as easily as those of a ballistic galvanometer. 

 Moreover, the behaviour of the mirror is very regular, and 

 its motion perfectly aperiodic. The mode of action of the 

 apparatus described above can be verified by observing the 

 motion of the mirror and the pointer under a low-power 

 microscope. 



One valuable feature of the apparatus is that it is quite 

 unaffected by any echoes of the original sound of impact 

 from the walls of the laboratory-room in which the expe- 

 riments are made. This is because the pointer attached to 

 the mica disk ceases to touch the mirror long before the 

 echoes from the walls arrive at it. The results obtained by 

 its use have been verified by working at different points 

 within the room, and also in rooms of widely differing shape 

 and size. 



In order to fully understand the action of the mica disk 

 and pointer, we have to study their forced vibrations under 

 the influence of the sound-pulse. We shall presently come 

 to this point. Meanwhile the results obtained by its use may 

 be described. 



Observations have been made of the deflexions shown by 

 the apparatus when two balls impinge directly upon each 















Fig. 



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20 40 60 80 100 120 



Inclination fin degrees) of tne line of collision 



140 ISO ISO 



tvith axis of horn — » 



other with a given velocity in different directions with respect 

 to the axis of the receiving horn. The results are recorded 

 in fig. 1. The curve exhibits quite a number of remarkable 

 peculiarities. It shows that the intensity is maximum in the 



