Fig. 3. (a) Experimental study of the jet velocity. 20 mg mL^–1 PVP solution processed at V = 9 kV for different values of d. The needle-collector distances of 16 cm, 20 cm and 26 cm correspond to an average velocity value of 2.6 m s^–1, 2.2 m s^–1 and 1.6 m s^–1 respectively, with relative errors of 0.4%. (b) Instability average velocity v _m of the jet vs. d.