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XXXVIII. Mechanical "Resonators " uncle)' Double Forcing. 

 By Prof. E. H. Barton, F.R.S., and H. M. Browning, 

 M.Sc* 



[Plates V. & VI.] 



FOLLOWING up the idea of a recent paper on Forced 

 Vibrations f, it seemed desirable to use a similar set 

 of pendulum " resonators " to exhibit experimentally the 

 effects upon them of two simultaneous harmonic forcings of 

 different periods. Four sets of photographs of six each 

 were taken. The first set had resonators highly damped. 

 The second set had denser bobs and so less damping. In 

 each case the two drivers differed greatly in their periods. 

 The third set of six photographs had the less damped reso- 

 nators and drivers whose periods differed slightly and were 

 made to approach and finally coalesce. The final set of six 

 had but one driver and was taken to demonstrate that the 

 resonators could discriminate several periods of driver between 

 those of adjacent resonators. 



Experimental Arrangements. — Fig. 1 shows the experi- 

 mental arrangement used throughout. Twenty " resonators }i 



Fig. 1. 



were made as pendulums with small paper cones as bobs and 

 black threads as suspensions. These cones were used alone 

 for the first set of six photographs, and afterwards with rings 

 of copper wire on them to lessen their damping in the sub- 

 sequent photographs. Their logarithmic decrements in the 

 two states were respectively of the order 0*1 and 0*025 per 

 half wave, but varied slightly from end to end of the 

 system. 



The pendulums J K were hung at equal distances and 

 their lengths arranged so that a line drawn through the bobs 

 pointed to A, one of the fixed ends of the cord. The extreme 

 lengths of these responding pendulums were about as three 

 to one. The direction of this cord AC pointed to the lens of 

 the camera. At the point C on this cord ACB was hung a 



* Communicated by the Authors. 



t Phil. Mag-. [6] vol. xxxvi. pp. 109-178 (Aug. 1918). 



