The model 309 Digital Earth Resistance Meter is a small, compact, battery powered, professional meter. This easy-to-use meter is invaluable to electricians and contractors who need to ensure the "ground" quality and effectiveness of buildings, structures, equipment or electrical systems. A good earth ground is required for new buildings or structures needing to pass required electrical codes. Older buildings can lose a good, effective earth ground connection over time. This can happen after a building or structure has been struck by lightning.