Laboratory of Electrification of Industrials Enterprises (LREEI) of boumerdes and CDTA have developed an in-house Helmholtz coil. It will be used as a source of external magnetic field for studying the Negative Bias Temperature Instability (NBTI) degradation mechanisms that can influence Metal Oxide Semiconductor (MOS) devices. This paper describes the different phases of design and construction of a Helmholtz coil controlled by a computer. Structure geometry, number of turns in winding and power supply calculations were done. The two windings are connected in series and the magnetic field was measured at different points. The results obtained were satisfactory. The magnetic field is uniform over an approximate length of 22 cm.