A particularly simple application of an original oscillation control system to a low-frequency torsion pendulum (equipped) for non-automatic discontinuous measurement of internal friction by the free decay method) has made possible the automatic continuous record of Q^-1 as a function of time at practically constant amplitude. Q^-1 can be measured up to at least 10^-2. The precision is typically to better than 3 × 10^-5 + 0.5%. The method also allows period measurement within less than 0.1 msec at 1 c/s. Some results on the relaxation effects due to about 0.002 wt% of dissolved nitrogen in α-Fe are reported, as an example of the instrument performance. Systematic errors of thermocouple temperature measurement in non-stationary conditions are discussed.