The minimum voltage for spark breakdown to occur across a material of given thickness held between electrodes producing a uniform electric field under specified test conditions. Expressed as volts per unit thickness. As the potential across any material is increased, Ohm’s law is typically obeyed initially, followed by a region of non-Ohmic behavior; finally a potential is reached at which the current increases at an uncontrolled rate. The corresponding field is the breakdown strength, also known as breakdown field, dielectric strength, and electric strength. Insulator breakdown strengths are typically 106 to 109 V/m at room temperature, varying considerably with test method. High vacuum has the highest breakdown strength followed by thin homogeneous solids and then liquids.