440 KENNELLY, TAYLOR— PROPERTIES OF 



justable, by means of a clamping screw ti', between lo and 40 cm., 

 with a corresponding oscillation frequency range of 0.6 to 0.3 '—'. 

 The logarithmic decrement being very small, it oscillates about the 

 suspension-wire axis with but little diminution of amplitude for 

 several minutes. In the model used, the time constant, or the time 

 of fall to i/eth amplitude, is 216 seconds. This driving cylinder 

 takes the place of the A.C. generator in the case of the telephone. 

 The smaller solid brass cylinder A\ has a mass of 26.8 gm. and a 

 moment of inertia of 8.4 gm.-cm.-. It is suspended from the driv- 

 ing cylinder by a smaller wire of copper, having a length usually 

 kept constant at 23 cm., and of 0.13 mm. diameter. This driven 

 cylinder corresponds to the telephone diaphragm, and performs 

 oscillations of the period impressed on it by K,,. The driving cyl- 

 inder Kq is so much larger than the driven cylinder /v^, that the 

 reaction of the latter is insignificant. With K^ and /v^ in action, 

 all the dynamical phenomena of the telephone diaphragm and its 

 velocity circle can be observed, the frequency impressed by /v^ being 

 adjusted by successive steps through a considerable range, by shorten- 

 ing up the main suspension wire Lq. As the impressed frequency 

 overtakes and passes the natural frequency of /v^, with its suspen- 

 sion Lj, the phenomena in the vicinity of resonance are reproduced. 

 The K^ system has its )>!, r, and s, in substantially the same manner 

 as the oscillographic systems described in Appendix I. 



Technique of Model. — In operating the model, the cylinder K^ 

 is first set by hand in torsional oscillation about the suspension-wire 

 axis, with the lowest frequency of longest suspension Lq, and with 

 as little side swing as possible. The initial angular amplitude of K^^ 

 may be made 90° or more. After a few oscillations, the coupled 

 pendulum system settles down to a substantially steady state, the 

 oscillations of K^ having the same frequency as those of K^. The 

 oscillations are allowed to subside naturally under the damping con- 

 stant (A = 0.00462 hyps, per second) until a convenient standard 

 amplitude is reached, as is indicated by a pointer P^, upon a suitably 

 supported angular scale 5"o. When this happens, the eye of the 

 observer at O can observe also the angular amplitude of K^ as well 

 as the angular phase-difiference of the two elongations at P^^^ and 



