Trivalent americium has a non-magnetic ($J$ = 0) ground state arising from the cancelation of the orbital and spin moments. However, magnetism can be induced by a large molecular field if Am$^{3+}$ is embedded in a ferromagnetic matrix. Using the technique of x-ray magnetic circular dichroism, we show that this is the case in AmFe$_2$. Since $〈 J_z 〉$ = 0, the spin component is exactly twice as large as the orbital one, the total Am moment is opposite to that of Fe, and the magnetic dipole operator $〈 T_{z} 〉$ can be determined directly; we discuss the progression of the latter across the actinide series. ; Comment: 6 pages, 5 figures to be published in Physical Review Letters (2015)