There are two coefficients of proportionality in this linear dependence of the energy split on the applied magnetic field (as shown in Figure 1). One is called the Bohr magneton β, which is a combination of fundamental constants and therefore is the same for all paramagnetic species. The other one is called the g-value. It is paramagnetic species specific and can vary significantly for paramagnetic species of different nature. For example, the g-value of free radicals is close to the value of 2, whereas the g-value of paramagnetic metals, for example of iron in haemoglobin, can be as high as 6.
Thus, the energy split in the figure above, when an external magnetic field H is applied, is △E = gβH.
The animation below shows what happens when we start increasing the external magnetic field while continuously irradiating the sample with an electromagnetic radiation. The frequency of the radiation ν is kept constant (in the microware radiation range), which makes the energy of the radiation quanta hν constant, too.