The more free ions the water contains, the better it can conduct electricity, i.e. the conductivity gets higher. This means that seawater has higher conductivity than lake water. Originally, the salinity in the sea was defined as the amount (g) salt per kilo of seawater. Today, the salinity is usually determined by measuring the conductivity of the water.
The conductivity of seawater can be measured with a salinometer, either in a water sample or directly in the ocean
The salinity of ocean water is 35 (‰) (‰ = promille). In the world's largest brackish body of water, the Baltic sea, the salinity of the surface water varies from 2 to about 18 (‰). The salinity in Norwegian seas varies between 30 and 35 (‰). Near the coasts, especially outside of river outlets, the salinity of the surface water can be significantly lower.
A hand refractometer can be used if the investigation only requires a rough estimate of the salinity in whole per mil's. This instrument instantly tells the salinity of a drop of water that is placed on it. It is relatively inexpensive, easy to handle and can be used in the field. In research and routine sampling, salinity, temperature and depth is measured with a CTD-system (conductivity, temperature, depth).