Automatic water feeder is also known as the communicator, the principle of the communicator can be explained by liquid pressure. If a U-shaped glass tube is filled with the same liquid, a small liquid sheet AB is assumed to be in the center of the bottom of the communicator. Let's say the liquid is stationary. The pressure of the liquid in the left tube to the right of tablet AB must be equal to the pressure of the liquid in the right tube to the left of tablet AB.
Because the connecting device is loaded with the same liquid, the density of the left and right two liquid columns is the same, according to the liquid pressure formula P=ρgh, only when the height of the liquid column on both sides is equal, the pressure of the liquid column on both sides of the liquid sheet AB can be equal. Therefore, in the case of liquid does not flow, the liquid level in each container of the communicator should be kept flat.