Alternatively, we can set up an experiment in which the system's behavior depends on the energy level. If we apply an external field of controlled frequency, we can measure the level separation by noting at which frequency a qualitative change happens: that would mean that at this frequency, the transition between two states has a high probability. An example of such an experiment is a variation of Stern–Gerlach experiment, in which magnetic moment is measured by finding resonance frequency for the transition between two spin states.