The principle of voltage measurement with multimeter
The ammeter is made according to the action of the magnetic field force on the current-carrying conductor in the magnetic field. There is a permanent magnet inside the ammeter, which generates a magnetic field between the poles. There is a coil in the magnetic field. There is a hairspring spring at both ends of the coil. Each spring is connected to a terminal of the ammeter. The spring and the coil are connected by a rotating shaft. On the front of the ammeter, there is a pointer. When a current flows, the current passes through the magnetic field along the spring and the rotating shaft, and the current cuts the magnetic induction line, so the coil is deflected by the force of the magnetic field, which drives the rotating shaft and the pointer to deflect. Since the magnitude of the magnetic field force increases with the increase of the current, the magnitude of the current can be observed through the degree of deflection of the pointer. This is called a magnetoelectric ammeter, which is the kind we usually use in the laboratory. The voltmeter can be understood as a large resistor in series with the ammeter. The internal resistance Ro of the ammeter is very small and can be ignored, and the external resistance R is very large. In this way, U=I ( R+R0 ) ~I*R is obtained according to Ohm's law, which is to measure voltage.
