Arcs and Atomization

Abstract

The running of an arc was first observed by striking a discharge between two horizontal carbon rods, and as this took on a bowed shape due to the rising current of hot air, the arc got its name, and kept it even when it became vertical or ran in another gas; we still speak of it as ‘burning’ although only slight oxidation may occur. The electrode connected to the positive pole, the anode, becomes hotter than the cathode if their diameters are equal, and for most analytical work samples are placed in a crater on the anode. The temperature profile across the arc shows a central core (usually 2–4 mm diameter) considerably hotter than the surrounding mantle, which is also hot enough to be luminous, and has about three times as great a volume. In the arc chemical reactions may occur, which may either continue or reverse changes in the stage of combination of the elements leaving the crater. These reactions are briefly discussed in Section 1.3 and in greater depth for certain of the elements separately. The rate of evaporation of the element being analysed depends on (i) whether it evaporates as an element or a compound, (ii) on the temperature difference between its boiling point and that of the arc crater, and (iii) on the speed of evaporation of other substances present with it in the crater.