A positively charged ball hangs from a silk thread. We put a positive test charge $q_o$at a point and measure $\frac{F}{q_o}$, then it can be predicted that the electric field strength E, before placing test charge is

Because of the presence of positive test charge $q_0$ in front of positively charged ball, charge on the ball will be redistributed, less charge on the front half surface and more charge on the back half surface. As a result of this net force F between ball and point charge will decrease