The part in which the current is to be induced during inductive heating cannot really be regarded as part of the heating machine (the analogy breaks down here). Rather, this part is the workpiece which needs to be heated, hardened, welded or melted.
The magnetic field penetrates the workpiece, with the depth of penetration depending on the material, temperature and frequency of the excitation current. Since the material and the desired temperature of the component are specified for a given process, only the frequency remains as a means to control the depth. The higher the frequency, the lower is the penetration depth of the field and, in turn, of the induced current and power. As a result, the outer area becomes hot first, and the heat then flows inwards by heat conduction due to the temperature difference. The lower the selected frequency, the deeper the penetration of the field and the greater the heat generated directly at depth.
This effect is used for different heating strategies, depending on the requirements of the workpiece. If a workpiece is only to be heated at the the edge, it is briefly heated at a high frequency with a high power density (see image on the left). If it is to be warmed up as evenly as possible, the frequency and power density used are lower and the time taken is somewhat increased (see image to the right).