160 Mr. A. Campbell on the Measurement of the 



tuned until it is set into strong vibration, and then the 

 frequency is calculated from the tension, length, and mass of 

 the wire. The original arrangement has been simplified, I 

 believe, by Mr. Alexander Russell, of Faraday House, who 

 uses a steel wire thrown into vibration by an iron-cored 

 choking-coil placed near it. In both arrangements some 

 uncertainty is introduced by the wire passing over a bridge 

 or pulley. 



In my first type of instrument I developed the last- 

 mentioned arrangement. The working parts of the instru- 

 ment are shown in fig. 1 : — 



Fig.l. 



A steel wire, W, is fastened at one end to a spring, S, and 

 at the other to a rack, R, sliding in guides. Near the wire 

 is fixed an electromagnet M with a laminated core ; this 

 magnet is excited by the given alternating current. By 

 turning the pinion P the wire can be tightened until it is 

 thrown into vigorous vibration by the magnet. The pointer Q 

 attached to the pinion then shows directly the frequency on a 

 suitably graduated scale. Many varieties of this arrangement 

 may be used ; for example, the pointer may be fastened to S, 

 or other kinds of magnifying-gear may replace the rack-and- 

 pinion. 



With careful use I believe that this type is accurate to 

 within less than 0*2 per cent., but it is not so convenient as 

 the second type, which I now proceed to describe. This more 

 practical instrument is shown diagrammatically in fig. 2. 



Here M is a choking-coil fixed near a steel strip A*, which 

 can be moved back and forward through the clamp B by a 

 rack-and- pinion, not shown. A's change of length is 



* I find, since writing the above, that in 1889 Professors Ayrton and 

 Perry suggested the use of a variable magnetic tongue near an alternating 

 electromagnet. Their idea, however, was never embodied in a diiect- 

 leadlng practical instrument. 



