Wire
noun, の adjective
A needle made of gold is a wire. Perhaps wires were made from gold mostly back in the day? And they were thin like needles? That's my guess.
The reading is the kun'yomi reading of both. You've learned 針 already, but 金 is more like お金, rendaku'd.
針金が床に落ちていたよ。
The wire was on the floor, you know.
針金を使って留めましょう。
Let's use a wire to fasten it.
これはハンガーの針金を曲げて作ったアートです。
This is art made by bending hanger wire.