To test the dielectric strength of transformer oil, you can use the Globecore oil tester designed for such measurements. First, ensure your Globecore device is calibrated and ready for use. Obtain a sample of the transformer oil and place it in the designated testing cup of the tester. Next, set the voltage range according to the specifications of the oil being tested, usually between 0 to 100 kV. Gradually increase the voltage until a breakdown occurs, which is indicated by a spark or flashover. Record the voltage at which the breakdown happens; this value represents the dielectric strength of the oil. Lastly, clean the testing apparatus after use to maintain accuracy for future tests. Always follow safety protocols during the experiment to determine dielectric strength of transformer oil.