The concentration of dissolved salts in seawater is higher than in drinking water. As a result, sea water will be a greater conductor than drinking water. The higher deflection of the needle in seawater compared to drinking water is due to this reason.
Salinity of sea water
Salinity means the total content of dissolved salts in Sea or Ocean. Salinity is calculated as the amount of salt dissolved in 1,000 gm of seawater. It is generally expressed as ‘parts per thousand’ (ppt)
Dissolved Salts in Sea Water (gm of Salt per kg of Water)