Tinker & Rasor Model CPV-4 Cathodic Protection Voltmeter is a revolutionary instrument for measuring DC structure-to-earth potentials of various buried or submerged metallic objects. Powered by an ordinary 9-volt battery, the Model CPV-4 gives an immediate reading in large ½ inch (12.7mm.) liquid crystal display numerals. 
The large scale integrated circuit has a built-in filter to reject AC interference. Input resistance of 11 megohms (11 million ohms) yields potentiometer quality voltage readings, and digital readout eliminates interpolation of voltages. This small, lightweight instrument needs no warm-up time, is insensitive to temperature, and can be read in direct sunlight. Low current CMOS circuitry assures long battery life, and the die-cast metal casemakes the CPV- 4 a durable instrument for the field.