PURPOSE This test is to measure the resistance offered by the insulating members of a component part to an impressed direct voltage tending to produce a leakage of current through or on the surface of these . members. A knowledge of insulation resistance is important@ even when the values are comparatively e high@ as these values may.be limiting factors in the design of high-impedance circuits. Low insulation resistance@ by permitting the flow of large leakage currents@ can.disturb the operation of circuits intended to be isolated@ for example@ by forming feedback loops. Excessive leakage currents can eventually lead to deterioration of the insulation by heating or direct-current electrolysis. Insulation-resistance measurements should not be considered the equivalent of dielectric withstanding voltage or electric breakdown tests. A clean@ dry insulation may have a high insulation resistance@ and yet possess a mechanical fault that would cause failure in the dielectric withstanding voltage test. Conversely@ a dirty@ deteriorated insulation with low insulation resistance might not break down under a high potential. Since insulating members composed of different materials or combinations of materials may have inherently different insulation resistance@ the numerical value of measured insulation resistance cannot properly be taken as a direct measure of the degree of cleanliness or absence of deterioration. The test is especially helpful in determining the extent to which insulating properties are affected be deteriorative influences@ such as heat@ moisture@ dirt@ oxidation@ or loss of volatile materials.