| The sine galvanometer, first described by Prof. Claude
Pouillet of Paris in 1837, is a precursor of the tangent
galvanometer. Like the tangent galvanometer, the sine galvanometer has
a coil of wire carrying the current to be measured, and a magnetic compass
needle in the middle of the coil.
The coil of wire is first oriented in the magnetic north-south direction and the compass needle is acted on only by the horizontal component of the magnetic field of the earth.. The passage of the current sets up a magnetic field perpendicular to the coil, and the compass needle points in the direction of the total magnetic field. The coil is now rotated through an angle Ø until the plane of the coil coincides with the needle. Under these conditions, the two torques acting on the needle are due to (a) the magnetic field B of the coil, which is at right angles to the coil, and (b) the component of the earth's magnetic field perpendicular to the coil: B'sinØ. These two torques just balance each other. Since the value of B is proportional to the current I setting up this field, I is proportional to sinØ.
The advantage of this apparatus is that the needle does not have to lie in a uniform magnetic field, and so the compass needle can be relatively large.
This apparatus in the Garland Collection at Vanderbilt University was bought from Ruhmkorff of Paris about 1875.