Direct deflection method:
The figure shows the measurement of high resistance using direct deflection method. For measurement of high resistance such as insulation resistance of cables, a sensitive galvanometer of d’Arsonval type is used in place of the microammeter. In fact many sensitive type of galvanometers can detect currents from 0.1-1nA.therefore, with an applied voltage of 1kV, resistance as high as 10^12 to 10*10^12Ω can be measured.
The first figure shows the direct deflection method for measurement of high resistance having metallic sheath. The galvanometer G shows the current between the conductor and the metallic sheath. The leakage current is carried by the guard wire wound on the insulation and therefore does not flow through the galvanometer as shown in figure.
Cables without metal sheaths can be tested in a similar way if the cable, except the ends or ends on which connections are made, is immersed in water in a tank. the water and a tank then forms the return path for the current .the cable is immersed in slightly saline water for about 24 hours and the temperature is kept constant at about 20 degree Celsius and then the measurement is taken as shown in second figure
The insulation resistance of the cable is given by,
In some cases, the deflection of the galvanometer is observed and its scale is afterwards calibrated by replacing the insulation by a standard high resistance (usually 1MΩ), the galvanometer shunt being varied, as required to give a deflection on the same order as before.
In tests on cable the galvanometer should be short-circuited before applying the voltage. The short-circuiting connection is removed only after sufficient time is elapsed so that charging and absorption currents cases to flow. The galvanometer should be well shunted during the early stages of measurement, and it is normally desirable to influence a protective series resistance (of several megaohm) in the galvanometer circuit. The value of this resistance should be subtracted from the observed resistance value in order to determine the true resistance. A high voltage battery of 500V emf is required and its emf should remain constant throughout the test.