Ramo's theorem provides a convenient way of simulating the currents induced on the electrodes of a radiation detector by the movement of charge carriers. The concept of weighting potential or of weighting field is the key ingredient in the calculation of the induced currents. The effect of the space charge present in the detector on the weighting potentials is discussed. Three types of space charge are considered and are shown to have different effects on the induced currents. These are 1) the polarization space charge, 2) a fixed volume space charge and 3) the charge stored in the depletion layer of a semiconductor detector. A new definition of the weighting potential is proposed that is valid for all situations.