352 Measurem&nt of the Period of a Reed or Tuning-forh. 



attached. In my apparatus the reed moves in a horizontal 

 plane, and so its plane is at right angles to that of the vibrating 

 rod attached to the axle of the rotating disk. Therefore the 

 image of a spot of light seen by reflexion in both mirrors (viz. 

 that attached to the rod on the axle, and that attached to the 

 reed) will for synchronism be drawn out into some form of 

 ellipse, and the permanence of the ellipse is an exact test of 

 equality between the vibration-period of the reed and the 

 period of revolution of the rotating-apparatus. 



(ii.) Determination of the Vibration-period of the Reed hy a 



Morse Receiver. 



Opposite the pair of springs Sj connected with the electro- 

 magnet M and concerned with the maintenance of the reed in 

 vibration, there is another pair of springs 83, similar in every 

 respect. One of these is connected through a Morse receiver 

 with a battery of the requisite electromotive force, the other 

 being connected with the other pole of the battery. When 

 the reed is in vibration it makes and breaks contact between 

 this pair of springs ; and we have, therefore, on the tape of 

 the Morse instrument a series of dashes ; a dash and the blank 

 space between it and the next corresponding to one vibration 

 of the reed. 



On the same tape, side by side with the track of the inking- 

 wheel of the Morse instrument, a pen is made to record the 

 movement of the pendulum of the standard clock in a suffi- 

 ciently ordinary manner. We have then merely to count the 

 number of dashes in a given time, and we have the number of 

 vibrations in that interval, and hence the pitch of the reed. 



The electrical connexion is made in my laboratory by the 

 escapement- wheel of the clock ; and hence for accurate result 

 the time taken must be an integral multiple of a minute^ the 

 period of revolution of the wheel. 



This method of determining the vibration-period of a reed 

 or tuning-fork has seemed to me to be of extreme simplicity, 

 and by taking a sufficient interval of time may be made of 

 any desired accuracy. A Morse receiver will spin out its tape 

 at a speed of 40 feet a minute, and the ordinary rolls of tape 

 will last for at least 15 minutes when it is doing so. 



The limit of accuracy in this method of determining the 

 rate of vibration is, I find at present, the constancy of vibra- 

 tion of the reed. It has been a surprise to me to find that the 

 vibration-period is not constant to much more than 1 part in 

 1000. 



