Workshop Technology
The purpose of a riser is to

Act as a reservoir for the molten metal
Deliver the molten metal from pouring basin to gate
Feed the molten metal to the casting in order to compensate for the shrinkage
Deliver molten metal into the mould cavity

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Workshop Technology
For arc welding

Alternating current with high frequency is used
Any one of these
Direct current is used
Alternating current with low frequency is used

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