SM004     Astatic Needle Galvanometer      circa 1940     Electricity

This type of current measuring instrument is used where an improvement in sensitivity over the normal tangent galvanometer is required.   This can be achieved by making the radius of the coil smaller or by putting in more turns of wire.   Both of these are more easily achieved when two magnetic needles are suspended such that opposing poles lie over each other, but the coil surrounds only one needle.   So, while the restoring couple due to the earth’s field is small owing to the opposing forces on the needles, the deflecting force is large since it acts on one strongly magnetized needle.   The construction of the instrument also makes possible a smaller diameter coil with a larger number of turns for the same resistance of a tangent galvanometer.

It must be kept in mind that this type of instrument has no simple law that connects the deflection with the current passing.   The response must be determined by calibrating each instrument separately.