Fig. 516.--Tangent galvanometer. It consists of a short magnetic needle suspended at the center of a coil of large diameter and small cross section. In practice, the diameter of the coil is about 17 times the length of the needle. If the instrument be so placed that, when there is no current in the coil, the suspended magnet lies in the plane of the coil, that is, if the plane of the coil be set in the magnetic meridian, then the current passing through the coil is proportional to the tangent of the angle by which the magnet is deflected from the plane of the coil, or zero position--hence the name: "tangent galvanometer."
The Tangent Galvanometer.--It is not possible to construct a galvanometer in which the angle (as measured in degrees of arc) through which the needle is deflected is proportional throughout its whole range to the strength of the current. But it is possible to construct a very simple galvanometer in which the tangent of the angle of deflection shall be accurately proportional to the strength of the current.
Fig. 517.--Horizontal section through middle of tangent galvanometer, showing magnetic whirls around the coil and corresponding deflection of needle.
Fig. 518.--Diagram of forces acting on the needle of a tangent galvanometer.
A simple form of tangent galvanometer is shown in [fig. 516]. The coil of this instrument consists of a simple circle of stout copper wire from ten to fifteen inches in diameter. At the center is delicately suspended a magnetized steel needle not exceeding one inch in length, and usually furnished with a light index of aluminum. When the galvanometer is in use, the plane of the ring must be vertical and in the magnetic meridian. A horizontal section through the middle of the instrument is shown in [fig. 517]. For simplicity, the coil is supposed to have but a single turn of wire, the circles surrounding the wire representing the magnetic lines of force. By extending the lines of force until they reach the needle, it will be seen that with a short needle, the deflecting force acts in an east and west direction when the galvanometer is placed with its coil in the magnetic meridian.
If, in [fig. 518], ab represent the deflecting force acting on the N end of the needle, the component of this force that acts at a right angle to the needle will be
ab cos x