In nuclear magnetic resonance (NMR) experiments the free induction decay (FID) current is induced by the precession of the excited magnetization, and the FID signal carries energy. This energy must come from the spin system, but has never been investigated quantitatively. This letter finds a way to calculate the energy transformed from the magnetic potential energy in the spin system to the electric kinetic energy associated with the FID current by revisiting the radiation-damping-modified Bloch equations. The result shows that under strong radiation damping conditions, almost all of the energy stored in the excited spin system is transferred to FID with negligible energy loss.

在核磁共振（NMR）实验中，自由感应衰变（FID）电流是由激发磁化的进动感生的，并且FID信号携带能量。该能量必须来自自旋系统，但从未进行过定量研究。这封信找到了一种方法，可以通过重新研究辐射衰减修饰的Bloch方程来计算从自旋系统中的势能转换为与FID电流相关的动能的能量。结果表明，在强辐射衰减条件下，几乎所有存储在受激自旋系统中的能量都以几乎可以忽略的能量损失转移到FID。