Why did the Yablochkov candle use alternating current?

The Yablochkov candle utilized alternating current to ensure that the two parallel carbon rods were consumed at equal rates.1

In this arc lamp, the rods were separated by a nonconducting porcelain clay partition that vaporized as the arc burned. Because direct current would cause one electrode to burn away faster than the other, alternating the polarity of the current allowed both rods to maintain a balanced consumption, which was essential for the lamp's operation as a practical lighting device.21