Lord Hayleigh : Acoustical Notes. 331 



observation by intermittent view. In my use of it an elec- 

 trically maintained fork of large dimensions and of home 

 construction was employed. The fork was provided with 

 solid (platinum) contacts and made 64 vibrations per second. 

 At the ends of the prongs were blackened plates of thin 

 metal perforated with slits, so disposed as to be opposite to 

 one another in the equilibrium position. When the vibra- 

 tions were excited by one or two cells, there were 128 views 

 per second through the slits. 



Viewed past the fork, some of the circles of revolving 

 teeth appear nearly stationary. Usually two neighbouring 

 circles can be picked out, which appear to revolve slowly in 

 opposite directions. From these data the necessary informa- 

 tion is obtained in a way that need not be further explained. 

 It is thought that it may be of service to give a reproduction 

 of Gordon's drawing (fig. 5, Plate VIII.). Photographic- 

 copies can easily be made upon any desired scale adapted to 

 the shafts round which it is intended to mount them. Care 

 should be taken to effect the junction properly, so that 

 the circles of teeth are continued through it without 

 irregularity. 



Plionie WJieel and Commutator. 



- By the use of the phonic wheel, invented independently by 

 La Cour and myself *, the speed of revolving shafts may be 

 not merely compared with a fork but automatically governed 

 thereby. I have used this method for driving a commutator 

 of the kind required for passing a regular succession of 

 condenser charges through a galvanometer, as for example 

 in determining the ratio of the electrical units (velocity 

 of light). The contacts required are such that a piece A 

 in connexion with the insulated pole of the condenser 

 shall make contacts alternately with a piece B representing 

 the insulated pole of the battery and another C connected 

 to earth and to the other pole of the battery and condenser. 

 Of course the contacts must be good, and it is essential that 

 both be not made at the same time. 



As in Thomson and Searle's work |, the commutator was 

 of the usual type adapted to a revolving motion, except 

 that the cycle of contacts was repeated so as to occur twice 

 in each revolution. The developed form is shown in the 



* Phil. Trans. 1883, p. 295. ScientiEc Papers, i. p. 355 ; ii. 

 t Phil. Trans. 1890, p. 607. 



p. 179. 



