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Maintenance of Vibration by Periodic Field of Force. 131 



the fork-interrupter are simultaneously recorded on photo- 

 graphic paper *. This may be done, either at some stage in 

 the progressive change of vibration-type, or when the 

 motion reaches a final steady state. Six records obtained in 

 the course of the work are reproduced in Plate I. They 

 represent, respectively, cases in which the frequency of the 

 vibration is equal to, J, J, ^, -J-, and £ of the frequency of the 

 field. Excluding the first, which is of the usual type, these 

 records are typical of those secured at a fairly early stage of 

 the progressive change of vibration- form, the electromagnet 

 being placed at about J- of the length of the wire from one 

 end, and the point of observation being at a similar distance 

 from the other end. The upper record in each case, which 

 represents the vibration of the wire, shows a strong upper 

 partial having the same frequency as the field. Other 

 records (not reproduced) show that at a later stage the 

 partial having the same frequency as the field becomes 

 relatively less important than the others and is not then so 

 obvious to inspection. It is clear, especially from the first 

 two of the records reproduced, that the motion includes a 

 considerable retinue of upper partials. • This is not sur- 

 prising in view of the fact, that the field due to the electro- 

 magnet under the excitation of the intermittent current is 

 practically of an impulsive character, as already shown in 

 the paper quoted above (Phil. Mag. Jan. 1915). 



Theory (as will be shown below) indicates that the 

 ordinary forced vibration which is excited when the tension 

 of the wire is adjusted for resonance is not at all essential 

 to enable a vibration having a frequency equal to a sub- 

 multiple of the frequency of the field to be set up and 

 maintained. This has been tested in the following way : 

 two electromagnets are placed opposite different points on 

 the wire, one or the other of which could be excited at 

 pleasure. The first being placed opposite a point distant, 

 say, ^ of the length from one end, and excited, the tension 

 of the wire is carefully adjusted for resonance so that it 

 vibrates in two, three, or larger number of segments as 

 desired. The second electromagnet is placed exactly opposite 

 a node of this forced oscillation, so that, in accordance with 

 a well-known principle, it is incapable of maintaining a 

 forced vibration of the ordinary kind when fed with inter- 

 mittent current. It is observed that when the second 



* A method based on the optical composition of the vibrations of the 

 fork and of a selected point on the wire could, no doubt, be used for 

 the same purpose, as an alternative. 



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