Operating mode of the SKP. The blue circuit shows that the frequency synthesizer G1 transmits a 1 kHz signal to the electromagnet and receives a response signal from the isolated needle, amplified by the pre-amplifier (1). A phase shifting of this signal is compensated by lock-in amplifier LIA1. The integrator (2) regulates a balancing voltage until the amplitude of the needle signal is cleared to zero. The required voltage is converted into the SKP signal by the computer PC1. The height control is achieved by a second modulation. For this an additional electronic control circuit, shown in red, consisting of frequency generator (G2), lock-in amplifier (LIA2) and PC2, is set to keep the electrical capacitance between the SKP tip and the investigated interface constant, which corresponds to keeping the distance between them constant.