In arc welding, long arc length require higher volt. Does the current goes up or down?

Answer 1

If you are using a constant current welder like a MIG then this is true - current stays constant.

When using a stick welder longer arcs equal higher resistance. Using Ohm's law this would naturally lower the current.
Do this practical experiment - get out your welder and a clamp on ammeter start your arc and slowly make the arc longer while you watch your ammeter. Do this with both types of welder.
Viola! you have your proof.

Answer 2

Current depends upon the welding but constant for a given situation.

Answer 3

current should always remain constant