Such electronic devices use human body as an electrical circuit and measure the electrical impedance between the lip and a needle introduced into a tooth. The electronic device, connected by wires between the lip clip and this needle (Fig.5), detects the impedance changes that can be displayed for example on an LCD screen. When the needle tip reaches the apical constriction the measured impedance also reaches the known value of the resistance between oral mucosa and apical constriction, giving an acoustic or/and optic advertising