  The Kondo effect, deriving from a local magnetic impurity mediating
electron-electron interactions, constitutes a flourishing basis for
understanding a large variety of intricate many-body problems. Its experimental
implementation in tunable circuits has made possible important advances through
well-controlled investigations. However, these have mostly concerned transport
properties, whereas thermodynamic observations - notably the fundamental
measurement of the spin of the Kondo impurity - remain elusive in test-bed
circuits. Here, with a novel combination of a "charge" Kondo circuit with a
charge sensor, we directly observe the state of the impurity and its
progressive screening. We establish the universal renormalization flow from a
single free spin to a screened singlet, the associated reduction in the
magnetization, and the relationship between scaling Kondo temperature and
microscopic parameters. In our device, a Kondo pseudospin is realized by two
degenerate charge states of a metallic island, which we measure with a
non-invasive, capacitively coupled charge sensor. Such pseudospin probe of an
engineered Kondo system opens the way to the thermodynamic investigation of
many exotic quantum states, including the clear observation of Majorana zero
modes through their fractional entropy.
