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XI. The Maintenance of Vibrations by a Periodic Field of 

 Force. By C. V. Raman, M.A., and Ashutosh Dey *. 



[Plate I.] 



THE effect of a periodic field of force on the motion of a 

 body subject to its influence has been discussed by one 

 of us in a previous publication in this Journal t> one of the 

 results of outstanding importance noticed being the series of 

 special relations between the frequency of the field and that 

 of the steady vibration possible under its action. It was 

 shown that the motion is capable of being maintained when 

 its frequency is either equal to, or is £ or §-, -\, i, or £ of the 

 frequency of the field, that is, any submultiple of it, but not 

 when the frequency has any intermediate value. The 

 experimental work and theory published in that paper 

 related to the motion of a system with only one degree of 

 freedom, the period of free vibration of which is determined 

 entirely by the field J. Recently, when experimenting with 

 the electrically maintained vibrations of wires, we have 

 noticed certain interesting effects which may be classed 

 with the phenomena referred to above, but which merit 

 separate discussion in view of the fact that the system in 

 this case has a series of free periods of its own, quite 

 independently of the field. These will now be described. 



Experimental Method and Results, 



The present investigation relates to the vibrations of a 

 steel wire about 2 metres in length, stretched vertically 

 under an adjustable tension, and subject to the transverse 

 periodic force exerted by an electromagnet placed near some 

 selected point on it. The electromagnet is excited by an 

 intermittent current from a fork-interrupter of frequency 

 which in our experiments is generally 60 per second. The 

 forced vibrations (having the same frequency as the inter- 

 mittent current), which are usually excited in the first 

 instance, when the tension of the wire is adjusted for 



* Communicated by the Authors. 



t " On Motion in a Periodic Field of Force," Phil. Mag. January 1915,. 

 by C. V. Eaman, M.A. Also ; Science Abstracts/ 1914, p. 586, and 1915, 



P- 434 - 



X The vibrations studied in that paper were those of the armature of 

 a synchronous motor of the attracted-iron type when not in rotation, 

 under the influence of the magnetic field due to an intermittent current. 



Phil. Mag. S. 6. Vol. 34. No. 200. Aug. 1917. K 



