438 



NATURE 



[August i, 191 8 



experimental study, and grope after the ideal model 

 which shall represent exactly the electrical or other 

 phenomena in question. But, whatever the uses of 



^^^^HIPII^^ ' M^Ssi; jjrj^jip 



!''](;. I.— Coui)led peiiduluiiis. 



such models, certain it is that their design and study 

 have appealed to many eminent men of science. In 

 this connection it may suffice to mention Faraday, 

 Maxwell, Lord Kelvin, Lord Rayleigh, Sir Oliver 



the usefulness of such a model is much enhanced if 

 its vibrations leave traces. This is easily arranged 

 by letting the bobs carry funnels of sand under which 

 a blackboard moves uniformly at right angles to the 

 direction of vibration. In the portable apparatus 

 shown in Fig. i the pendulums are of the double- 

 cord type, and allow both traces to be obtained simul- 

 taneously and thus record the relation of amplitude 

 and phase for each pendulum. 



With this apparatus the coupling can be varied at 

 will, and easily adjusted to any desired value from 

 I per cent, to 60 per cent, or more. The greater the 

 droop of the bridles, the greater the coupling, the 

 quantitative relation being simple. It is noteworthv 

 that for equal bobs and pendulum lengths a 60 per 

 cent, coupling gives superposed periods as 2:1, just 

 as in the electrical case for equal periods. Indeed, 

 with any specified coupling the ratio of period is the 

 same for this mechanical case and for the electrical one. 

 The masses of the bobs and the lengths of the pen- 

 dulums are adjusted at pleasure, and the initial 

 conditions may be anything that* is desired. (Simul- 

 taneous traces with this apparatus were then obtained, 

 others exhibited, and photographs of a number thrown 

 on the screen. One set of traces illustrated the rapidly 

 damped vibrations of the quenched spark, and the 

 corresponding almost undamped vibrations on the 

 antenna in this system of wireless telegraphy.) 



With equal bobs and equal lengths, the coupling 

 being small, each pendulum exhibits in turn the same 

 maximum and the same minimum as the other. 

 With small couplings, equal lengths, but bobs as 

 20 : I, the case of forced vibrations is approached. 

 That is to say, the heavy bob loses but little ampli- 



^•ftf^ ^as3«f Le^^t^.^ 





Cr;% Wcus^p tencf, 



Fig. 2.— Vibration traces of coupled penjuluii 



Lodge, Sir Joseph Thomson, Profs. J. A. Fleming, 

 T. R. Lyle, of Australia, and W. S. Franklin, of 

 America. 



For either quantitative work or mere illustration 



NO. 2544, VOL. lOl] 



tude, while that of the light bob grows from zero to 

 its maximum. With bobs as 5 : i, the heavy bob loses 

 appreciably, while the light one proceeds to its 

 maximum. 



