Earth tester:-The resistance of the earth is measured with the help of earth tester. It consists of a hand cranked type generator, the magnet poles, crossed coils and flexible spirals. It consists of rotating current converter and a rectifier in addition to ohm meter. Both of these consists of simple commutator made up of L shaped segments, mounted on the generator and rotated at the same speed by the operating handle, each commutator has four fixed brushes in contact with it.
Operation: - It is connected to earth whose resistance is to be measured, and the other spike P and R, as shown in the figure. When handle is rotated the D.C. flows from the generator through the current coil of the movement to the current reverser, and alternating current from the reverser through the soil between the electrode E and R . This voltage drop between electrode P and E is rectified by the rectifier and fed to the potential coil of the meter. As the indication of the meter depends upon the ratio of the potential across its potential coil, and current passing through its current coil, the deflection of the pointer will indicate directly resistance in ohm of the earth under test.