248 



Professor Edwin H. Barton 



[March 8, 



pendulums are of the double-cord type and allow both traces to be 

 obtained simultaneously, and thus record the relations of amplitude 

 and phase for each pendulum. AVith this apparatns the coupling can 

 be varied at will and easily adjusted to any desired value from 

 1 per cent, to 60 or more. The greater the droop of the bridles the 

 greater the coupling, the quantitative relation being simple. It is 

 noteworthy 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 



n 



.•M 



Fig. 1.— Coupled Pendulums. 



ratio of periods is the same for this mechanical case and for 

 the electrical one. The masses of the bobs and the lengths of the 

 pendulums are adjusted at pleasure, and the initial conditions may 

 be anything that is desired. (Sinmltancous traces with this apparatus 

 were then obtained, others exhibited, and photographs of a number 

 thrown on the screen.) 



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 : 1, the case of forced viljrations is approached. That is 



