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As the power of the high voltage generators increased, the tube anode had to withstand higher temperatures. In the first example shown, the anode was an Aluminium disk. Although better than the glass end wall of the tube, the amount of power that the anode could handle was still low. In this tube, the anode is a square of Platinum foil. Since platinum has a much higher melting point than aluminium, the tube could handle more current. In addition, the higher the atomic number means that more energy is converted into X-rays.
On the down side, Platinum evaporates very readily, as can be seen in the photograph, where there is a noticeable mirror on the glass.