We know that no matter it is a solid, liquid, or gas, the matter is divided into the conductive matter, dielectric (insulating matter), and semiconductor matter according to its electrical properties.
For liquids alone, there are also conductive liquids and non-conductive liquids. In conductive liquid substances, there are a large number of charged free particles (electrons or positive and negative ions).
These charged free particles will move regularly under the action of an electric field, so they form a certain direction in the substance current. The conductivity of liquid metals, such as mercury and sodium, depends on the presence of so-called free electrons or conduction electrons in them, and the current in the liquid metal is formed when they move regularly.
Electrolytes (water, aqueous solutions, acids, alkalis, salt solutions) contain positive and negative ions, and their movement also forms an electric current. In liquids such as oil and some organic substances, there are only a very small amount of charged-free ions. Therefore, almost no electric current is formed, and polarization is the first phenomenon.
Water is an electrolyte. The relative permittivity of water is ε=81.5, which is the highest known permittivity. This is also the reason why the ionization power of water is so high.
Water’s conductivity is very small, but its behavior obeys Ohm’s law.
When the water contains dissolved salts, its conductivity increases and changes depending on the temperature.
It should be noted that the current flow of free electron conduction in the liquid metal fluid and ionic conduction in the electrolyte fluid are both forms of conduction current.
The current flow in the dielectric is only the displacement current and the current flow in the arm is like a capacitor. However, since the boundary between conductor and insulator is not obvious, there may be conduction current and displacement current in the electromagnetic field at the same time.
This phenomenon will appear when the electromagnetic flowmeter measures low-conductivity fluids.