A positively charged ball hangs from a long silk thread. Electric filed at a certain point (at the same horizontal level of ball) due to this charge is E. Let us put a positive test charge `q_(0)` at this point and measure `F//q_(0)` on this charges. then E

Explanation

When we try to measure the electric field at point P then after placing the test charge at P, it repels the source charge (suspended charge) and the measured value of electric field `E_("measured")=F/q_(0)` will be less then the actual value `E_("act")`, that we wanted to measure.