When a voltage is applied across a piece of insulator, the electric field at each point is equal to the gradient of the voltage. The voltage gradient may vary at different points across the object, due to its shape or local variations in composition. Electrical breakdown occurs when the field first exceeds the dielectric strength of the material in some region of the object. Once one area has broken down and become conductive, that area has almost no voltage drop and the full voltage is applied across the remaining length of the insulator, resulting in a higher gradient and electric field, causing additional areas in the insulator to break down. The breakdown quickly spreads in a conductive path through the insulator until it extends from the positive to the negative contact. The voltage at which this occurs is called the "breakdown voltage" of that object. Breakdown voltage varies with the material composition, shape of an object, and the length of material between the electrical contacts.