uninsulated live part or live lead the length of the testing part may be preset and is selected to be the length of the required minimum safe distance and if the free end of the testing part can contact with the charged body i e the uninsulated live part or live lead it 39 s demonstrated that the distance is less than or equal to the minimum safe distance and on the contrary if the free end can not contact with the charged body it 39 s demonstrated that the distance is greater than the minimum safe distance the distance conforms to the safety set standard when it 39 s greater than or equal to the minimum safe distance generally the testing part is a group of testing heads having different calibration length value the testing head can be exchangeably used on the sharp end 3 of the finger shaped rod 2 on occasions where the voltage levels are distinct there exist distinct requirements for the safe distance the testing heads are made to have different calibration length values and can be exchanged when in use such that that it 39 s possible to satisfy the test under different voltage levels as shown in fig1 b the sharp end 3 of the finger shaped rod 2 is provided with a partially spherical groove 15 the bottom of the partially spherical groove 15 is securely provided with a magnet 4 and the partially spherical groove 15 is provided therein with a connection ball 5 made of ferromagnetic materials the connection ball 5 contacts with the magnet 4 and is attached into the partially spherical groove 15 through.