1879.] 



On the Rate of Vibration of Tuning Forks. 



291 



III. " On the Determination of the Kate of Vibration of Tuning- 

 Forks." By Herbert McLeod, F.C.S., and George 

 Sydenham Clarke, Lieut. R.E. Communicated by Lord 

 Rayleigh, F.R.S. Received January 16, 1879. 



(Abstract.) 



The paper contains a description of some experiments made with 

 a view to determine the absolute pitch of tuning forks by means of 

 a method proposed by the writers in a previous paper (" Proc. Roy. 

 Soc," vol. xxvi, p. 162). 



It commences with a description of the time measurer adopted, 

 consisting of a compensated pendulum, worked by electricity, the 

 impulse being given by a driver depending for its action on gravity 

 alone. The pendulum is arranged to give second contacts, driving a 

 clock wheel with sixty teeth. This wheel has a platinum pin giving 

 minute contacts, but it is used merely as a switch, the circuit being- 

 closed by the pendulum itself. The current works a relay, and closes 

 the circuit required. 



The tuning fork apparatus consists of a brass drum resting on 

 friction wheels, and driven by a weight and train. Uniformity of 

 motion being of great importance, an air-regulator, consisting of a fan 

 enclosed in the lower compartment of a cylindrical box, is employed. 

 By means of a diaphragm and vanes the fan can be made to do more 

 or less work by pumping air from the lower into the upper compart- 

 ment. The fan spindle carries a pulley driven by a thread passing- 

 round the drum. 



Round one end of the drum are wrapped strips of paper on which 

 white equidistant lines Lave been so ruled that they are parallel to 

 the axis of the drum when the strips are in position. The strip most 

 frequently used has 486 lines round the complete circumference of the 

 drum. Opposite this graduated strip is placed a microscope with its 

 axis horizontal. In the substage is placed a 2" objective, producing 

 an image of the graduations at the focus of the object-glass of the 

 instrument. At the common focus of the two lenses is placed the 

 tuning fork, the stem of which is held vertical in a vice. The fork is 

 partially enclosed in a glass case, and is so adjusted that the image 

 of one of its limbs seems to cut the image of the graduations at right 

 angles. The fork is set in motion by a suspended double-bass bow. 

 If when the fork is in vibration the drum is made to rotate with such 

 a velocity that one of the graduations passes over the interval between 

 two adjacent graduations in the time of one vibration of the fork, a 

 stationary wave is seen of length equal to the length of that interval. 

 To determine the number of vibrations of the fork in a given time, it 



