The dielectric constant, also known as permittivity, is an index of the ability of a substance to attenuate the transmission of an electrostatic force from one charged body to another (i.e., reduce the electric field within the dielectric). The higher the value, the greater the attenuation. The standard measurement apparatus utilizes a vacuum whose dielectric constant is 1. In reference to this, various materials interposed between the charged terminal have the following value at 20°C:

 

air 1.00058
glass 3
benzene 2.3
acetic acid 6.2
ammonia 15.5
ethanol 25
glycerol 56
water 81

 

The exceptionally high value for water accounts for its unique behavior as a solvent and in electrolytic solutions. Dielectric constant values decrease as the temperature rises.