The science of Jawari
It is in this field that a lot of scientific research has been made, to establish exactly what happens when the string vibrates on the rounded bridge.
It is interesting to know that experiments have been made on musical strings since 1625, when Mersenne proved that the tonal pitch was related to the length, tension and thickness of the string. Since then, experimentation has not yet fully explained what actually happens when a string vibrates. In fact this has proven to be a remarkably complex problem, and one which I can never hope to explain, as I am not a scientist and I do not claim to be one. However with the help and advice of Wim van der Meer, one of Europe's authorities on North Indian Classical music. He has himself made various experiments on jawari; and I can endeavour to explain, in a simplified way, what happens when a string vibrates on a tanpura bridge.
The tanpura bridge is basically flat and slightly convex (see photos). As the string vibrates over the bridge it alternates in length. When the string vibrates up, the string is at its longest and leaves the bridge at point (A). The contrary applies as the string vibrates down leaving the bridge at point (B). This minute alternating change in length allows us to hear distinct overtones. In the case of the note "A", the vibration alternates 440 times every second, in other words 440 hertz.
The Nobel Prize winner C.V.Raman, who did extensive studies on bowed strings, drew attention to the remarkable acoustic properties of the tanpura. He observed how the string vibrated over the bridge, and by inserting a thin thread (the "jiva") between the string and the bridge, a finely adjustable grazing contact of string and bridge is attained. The “jiva", when correctly positioned, intensifies the powerful series of overtones which creates a bright, full sound.