What is the principle of the electrometer?
Let’s assume that the body had a negative charge. Consequently, there was an excess of electrons on the body. When in contact with the ball, some of the electrons moved to the electrometer. In this case, the ball became negatively charged. Since the ball is connected to a rod, and that in turn is with an arrow, and they are all conductors, the electrons have moved to the rod and arrow. A plastic stopper is needed to isolate the ball, rod, arrow system. As a result, the rod and arrow received the same negative charge. Consequently, they will repel and the arrow will deflect. Moreover, the larger the charge, the greater the deflection of the arrow. The electrometer only allows you to estimate the amount of charge, i.e. say one body has a higher charge than another.