(a) The two STM tip measurement electrical circuit diagram used on the Pb(111) surface with its 2 virtual ground I–V converters and the back ground controllable switch connected to the sample holder. TheV_S sample holder is defined using µ_S  =  E_F +eV_S. (b) Three “one shot” dI/dV spectra recorded on Tip A while the far away Tip B is in atomic point contact with the Pb(111) surface to control the sample chemical potential from its surface. Tip B potential is V_B = 0.0 mV (black), V_B = +30 mV (blue) and V_B = –30 mV (red). Tip A was stabilized in a tunnelling regime with its STM Feed-back loop set up I_A = 200 pA, for V_A = 30 mV.