Weigh two light nuclei, fuse them, and weigh the result — it comes out LIGHTER. The missing mass didn’t vanish; it left as energy, E = Δm·c². That tiny deficit is the fire of the sun and the yield of the bomb. Slide through the reactions and watch mass turn into light.
A bound nucleus weighs LESS than its separate parts by the mass defect Δm; multiply by c² and you get the binding energy that holds it together — and that is released when the nucleus forms. For D + T → ⁴He + n the parts lose 0.0189 u, which is 17.6 MeV of energy (per fusion, millions of times a chemical reaction). The instrument uses real measured nuclear masses. A fail-loud self-check throws unless the deficit is positive and Δm·c² equals the known ~17.6 MeV yield of D–T fusion.
A few example reactions with tabulated masses; the binding-energy-per-nucleon curve (peaking at iron-56) is why light nuclei FUSE and heavy ones FISSION, both releasing energy. The E=Δmc² arithmetic is exact.