The Alternating Current Arc

The voltage drop across the ac arc reaches an initial maximum each half-cycle, followed by a rapid decrease and a constant low voltage until the half-cycle is completed. The high-voltage maximum triggers the discharge. The operation of the ac arc is therefore primarily low-voltage with a peak 

The ac arc is used primarily for liquid samples which can be evaporated on the flat ends of the electrodes. The selective volatilization that is a problem with the dc arc does not occur with the ac arc however, it is important to keep the gap distance constant in the ac arc. Variations in line intensity ratios occur when the electrode separation changes, due probably to temperature changes in the arc gap. 

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