The conductivity is the ability of water to pass an electrical current, and the dissolved ions are the conductors. The major ions in water are inorganic dissolved solids such as carbonate, bicarbonate, sulfate, chloride, nitrate, and phosphate anions (ions that are negatively charged) or sodium, potassium, magnesium, calcium, iron and aluminium cations (ions that are positively charged). Since conductivity varies with temperature, the warmer the water, the higher the conductivity, most measuring instruments automatically correct the readings to a standard 25°C. The electrical conductivity EC is the reciprocal of the electrical resistivity and is expressed in SI in Siemens per centimeter [S/m]. Since the Siemens is a very large unit, the micro Siemens (µS/cm) is typically used [1S/cm = 1.000.000µS/cm].