Introduction to the Structure and Principle of the Analog Multimeter
Inside the circular permanent magnet, there is a set of components supported by a cylindrical iron core. Outside the cylindrical iron core is a movable aluminum frame. Inside the aluminum frame, there is a coil made of fine enameled wire. Both ends of the coil are equipped with pointed pins, a hairspring, and a zero-adjustment rod. Its flexibility determines the sensitivity of the meter head, and it moves in a way similar to the pendulum of a watch. The two ends of the coil are a positive terminal and a negative terminal, with the positive pole (+) indicated by a red wire and the negative pole (-) indicated by a black wire.
It is mounted on an aluminum frame that can rotate around an axis. On the rotating shaft of the aluminum frame, there are two flat spiral springs and a pointer. The two ends of the coil are respectively connected to these two spiral springs, and the measured current enters the coil through the springs. Each of the two poles of the horseshoe-shaped magnet is equipped with a pole piece whose inner wall is in the shape of a cylindrical surface. Inside the aluminum frame, there is a fixed cylindrical iron core. The function of the pole pieces and the iron core is to make the magnetic field lines between them all run in the radial direction and be evenly distributed along the circumference.
When the coil moves in the magnetic field, no matter what position it rotates to, its plane is parallel to the magnetic field lines. When current passes through the coil, the two sides of the coil that are parallel to the axis are both affected by the magnetic field force. The effect of these two forces is to make the coil rotate. When the coil of the meter head rotates, the spiral spring is twisted, generating a force that resists the rotation of the coil, and the torque of this force increases as the rotation angle of the coil increases. When this resistance increases to the point where it cancels out the rotational effect of the magnetic field force, the coil stops rotating.
According to the principle of the magnetic field that like poles repel and opposite poles attract, once the magnetic field of the microampere-level coil is connected in reverse, the magnetic field it generates will become the opposite magnetic field, and they will interact with each other and deflect in the opposite direction. Moreover, the force exerted on the coil by the current passing through it is proportional to the current. Therefore, the greater the current in the coil, the greater the rotational effect of the magnetic field force, and the greater the deflection angle of the coil and the pointer. Thus, based on the magnitude of the pointer's deflection angle, the strength of the measured current can be determined. When the direction of the current in the coil changes, the direction of the magnetic field force also changes accordingly, and the deflection direction of the pointer changes as well. So, based on the deflection direction of the pointer, the direction of the measured current can be determined.
