MadSci Network: Physics

Re: Why can't a moving coil galvanometer be highly sensitive as well as accurat

Area: Physics
Posted By: Murshed Hossain, Senior Research Scientist, Physics, University of Delaware
Date: Thu Jul 10 15:36:48 1997
Area of science: Physics
ID: 868552975.Ph

Galvanometer is an instrument where a coil is placed in the magnetic field of a permanent magnet. When an electric current is passed through the coil it experiences a torque due to the interaction of the current with the magnetic field. As a result the coil and the needle (or the mirror) is deflected. The amount of deflection is a measure of the amount of current through the coil. For a given current, the more the deflection the more sensitive the galvanometer is. So, for measuring current accurately one must have a sensitive galvanometer. That is, one must have a detectable deflection for a very small current. So it seems that the question is wrong. But it is a legitimate question in the following context.

Let us say that we want to use a galvanometer to balance a Wheatstone bridge. That is, we are not particularly interested in knowing the amount of current through the galvanometer rather we want to measure the voltage difference across the galvanometer. Also assume that we have a given magnet and we can choose the number of turns in the coil of the galvanometer to make the instrument more or less sensitive. The amount of deflection for a given current goes up with the number of turns in the coil and thus make the galvanometer more sensitive. But now, by increasing the number of turns in the coil we have increased the resistance of the galvanometer. This increase in resistance makes the galvanometer less sensitive to voltage difference thus less accurate in trying to balance the bridge.

If this does not answer the question then I would have to know more about the situation where the question of compromise between the sensitivity and accuracy arose. I would be glad to reply to direct e-mails. [EOB]

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