The magnets are quite brittle. Don't let them fall on to hard surfaces (like concrete) from a height – they may shatter into sharp pieces. Straight copper pipe, at least 30cm long. 15 or 22mm ...

A copper pipe is a reddish-brown color ... Another way to know if you have a galvanized pipe is to bring a fridge magnet with you. If the magnet sticks to the pipe, it's galvanized.