Vibrations of a Drum (Rotating Drum Method)

Vibrations of a Drum 

The vibration of a drum or the rotating drum method is used to measure the frequency of an electrically maintained tuning fork . It is also known as Chronographic method for the measurement of frequency of a tuning fork .

We will discuss about the construction of its physical arrangement and its working procedure to measure frequency of tuning fork . So , let's get started -

Construction :- 

The physic arrangement of the whole rotating drum method is shown in the above figure (a) .

As shown in the figure (a) ,

1. A metal drum of about 4 to 5 inches diameter is placed which is able to be rotated on its groove N or can also be move in upward or downward direction with the handle H .

2. The surface of the metal drum is covered by a plane paper painted with carbon black .

3. A tuning fork is placed near the drum . It has a pointed sty T made up of aluminium is clamped at its one arm .

4. The tuning fork is placed in such a way that the knife edge of sty T is always in contact with the paper painted with carbon black (side surface of drum). 

5. There is a primary and a secondary circuit is created in this arrangement which is working on the principle of mutual induction .

6. The drum D and the tuning fork is connected with the terminals of a secondary coil S and hence a secondary circuit is formed .

7. The primary circuit consists of a second pendulum , a bowl C filled up of mercury and a battery .

8. In the secondary circuit , the second pendulum is arranged in such a way that its Bob B touches mercury two times in its one complete oscillation .

9. The primary circuit completes when the Bob B touches mercury and a strong electric current flows through primary circuit .

10. Due to mutual induction, a current is induced in the secondary coil results producing a spark between the edge of sty T and drum D . It burns a small portion of carbon black of the paper wrapped on drum D .

Working :- 

1. We start rotating the drum D with the help of handle H , along with starting the oscillations of Bob B .

2. The oscillations vibrates the tuning fork simultaneously.

3. When the Bob B touches the mercury , a spark is produced between T and drum D . This results a small portion of paper being burnt out .

4. The Bob completes one oscillation in two seconds , therefore we obtain the burnt portion on paper after every one second .

5. After this whole process , we remove the paper painted with carbon black from drum and spread it on the table . It will look like figure (b). 

6. Now , count the number of complete waves between two successive burnt portions using a microscope .

7. We know that the time difference between burnt portions is one second , therefore , the number of waves between it , will be the Frequency of the Tuning Fork used .

Drawbacks :- There is a sty T is connected on the tuning fork which effects in reducing its frequency .

So , thus was the information about the Rotating drum method or Vibrations of a drum method for the measurement of frequencies of an electrically maintained tuning fork .

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