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| Melde's Experiment |
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| Melde's electrically maintained tuning fork consists of a large tuning fork (F) made of a ferromagnetic alloy, whose shank is rigidly clamped to a heavy rectangular wooden board (W). A bent strip of a metal is attached to one of the prongs of the fork and this serves as a spring. This metal piece is in contact with a metal screw (S), which can be worked through a fixed nut. An electromagnet is fixed to the wooden board between the prongs. A current can be passed through the electromagnet by connecting the terminals provided on the board, in series with a battery (Ba), a plug key(PK) and a rheostat (Rh). When the plug key is inserted, a current flows in the circuit and the electromagnet draws the prongs towards it. This breaks the contact between the spring and the screw and the current stops. |
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| The electromagnet can no longer attract the prongs, which therefore, fly back to their original positions and the contact is again established. The prongs are again drawn inwards. This process repeats and the prongs vibrate. The frequency of vibration is a constant, depending on the parameters related to the fork. |
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| A long flexible thread is attached to the end of a prong and the thread is passed over a frictionless pulley (P) fixed near the edge of the table. A scale pan is attached to the end of the thread and weights can be placed in the pan to keep the thread taut. By making the thread parallel to the length of the prongs, transverse waves can be produced in the thread. If the thread is perpendicular to the length of the prongs, longitudinal waves will be set up in the thread. |
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| To determine the frequency of the electrically maintained tuning fork (transverse mode) |
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| Melde's fork is arranged such that transverse waves can be set up in the string, when it vibrates. The scale pan is removed from the string and its mass S is found in a balance. It is attached to the string again. The fork is made to vibrate by inserting the plug key and adjusting the contact screw. A suitable weight M is placed in the pan such that the thread is divided into a certain number of segments, say 8. |
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A known length of the specimen thread is weighed in a balance and its mass is determined. Thus m, the mass per unit length of the thread is calculated. The length L of the vibrating thread from the point where it touches the pulley, is measured with a metre
scale. If n is the number of loops formed, then  is
calculated, where T is the tension in the thread given by T = (M +S)g. |
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| The experiment is repeated for different number of loops formed on the thread by adjusting the mass M. We know that when the thread vibrates with n loops, the frequency of vibration is given by |
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| The readings are entered as follows: |
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| Mass of the scale pan = S =……………….. Kg |
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| Acceleration due to gravity = g = ………..ms-2 |
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| Length of the thread = L = …………………..m |
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| Length of the specimen thread = …………m |
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| Mass of the specimen thread = …………….Kg |
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 Mass per unit length of the thread = ….Kgm-1 |
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| Frequency of the electrically maintained tuning fork = |
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| = ………….. Hz |
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