In 1925, Uhlenbeck and Goudsmit showed that small discrepancies between Bohr's quantum description of the hydrogen atom and experimental values could be explained if the electron behaved as if it had angular momentum and a magnetic moment. They were proven correct, and intrinsic angular momentum is now one of the fundamental properties of a particle.
This elegant apparatus, designed by the physics department of Dartmouth College, lets the students measure the value of a single electron's magnetic moment. The parametric organic compound DPPH (diphrenyl-picri-hydrazyl) has an unbound electron that orients in an external magnetic ﬁeld. The electron spins like a top in the polarizing ﬁeld with a frequency proportional to the magnitude of the ﬁeld. If the polarizing magnetic ﬁeld is swept through a range, the electron's precession varies in frequency with the ﬁeld change. If students set up a weak high frequency ﬁeld at right angles to the primary ﬁeld, the weak ﬁeld will interact with the electron when their frequencies are the same. A pair of Helmholtz coils, powered by an ac current, set up the polarizing magnetic ﬁeld. A probe containing the DPPH is inserted into the ﬁeld. A tiny coil, which provides the inductance for the oscillator, surrounds the crystals and supplies the transverse, high-frequency signal. The coil mounts in a sturdy metal probe with an acrylic end cap. Students can see the coil, but it is well-protected. As the magnetic ﬁeld varies from positive to negative and back, the electron's frequency matches the oscillator's frequency at each particular ﬁeld value. The oscillator detects the energy transfer and displays it on the oscilloscope screen.
Measures the electron's precession frequency in a magnetic ﬁeld.
Intrinsic angular momentum of the electron.
Variation of frequency with ﬁeld strength.
Electron Spin Resonance Apparatus includes:
Power supply, Two Helmholtz coils, Probe and electronics, complete instruction manual