Measure of the polarity of a medium. The force (F) between two electric charges (e) at a distance (d) apart in a vacuum is expressed as: F = e²/d². In any other medium: F = e²/∊_rd² where ∊_r is the dielectric constant. Typical values are: 1.0 for air; 1.013 for steam; 15.5 for liquid ammonia; 80.36 for water at 20°C; usually 3.18 for ice, although at low frequencies it can be up to two orders of magnitude higher; 5–19 for granite; and 3–105 for dry to moist sand. The dielectric constant is temperature dependent, increasing as temperature increases, and strongly frequency dependent with low-frequency (less than 100 Hz) values being up to 30% higher than those of high frequency (more than 100 Hz). It is analogous to the magnetic permeability. The complex relative permittivity, ∊* is given by ∊* = ∊′ – j∊″ where ∊′ and ∊″ are the real component (relative permittivity) and the imaginary component (dielectric loss factor) respectively, and j = √ − 1.