The figure shows Prandtl's visualization of the "Tomlinson" mechanism.

The mechanism for atomic friction, presented in 1929 by Tomlinson was already used a year before by Prandtl as a model for the dynamic of shift lines in a crystal.

Prandtl was interested in the irreversible transformations of non-monocrystal solid. Because there is no hysteresis in ideal mono crystals between voltage and transformation, he was drawn to say, that the dynamics of shift lines must be responsible for this phenomenon. His model of the behaviour of an atom belonging to a shift line equaled the one of Tomlinson for a surface atom during the friction process. It's obvious that he could explain the hysteresis effects.