Vim li cas qhov tsis yog -tus cwj mem tiv tauj teeb thaum koj sim koj tus kheej?
Because he works on batteries. The simple test pen has no battery and relies on the test source's own voltage to light up the light. The advanced test pen that can measure weak current or whether it is powered or not has a battery inside. It is similar to a current amplifier, and it can measure how weak the current is. Different according to the range.
