  In 1971, Zel'dovich predicted the amplification of electromagnetic (EM) waves
scattered by a rotating metallic cylinder, gaining mechanical rotational energy
from the body. Since then, this phenomenon has been believed to be unobservable
with electromagnetic fields due to technological difficulties in meeting the
condition of amplification, that is, the cylinder must rotate faster than the
frequency of the incoming radiation. Here, we show that this key piece of
fundamental physics has been hiding in plain sight for the past 60 years in the
physics of induction generators. We measure the amplification of an
electromagnetic field, generated by a toroid LC-circuit, scattered by an
aluminium cylinder spinning in the toroid gap. We show that when the Zel'dovich
condition is met, the resistance induced by the cylinder becomes negative
implying amplification of the incoming EM waves. These results reveal the
connection between the concept of induction generators and the physics of this
fundamental effect that was believed to be unobservable, and hence open new
prospects towards testing the Zel'dovich mechanism in the quantum regime, as
well as related quantum friction effects.
