How do I determine how much current a wire can safely handle? I made some electromagnets with some coil and 9 volt batteries, but the wire and the batteries get real hot after a minute, which I assume is not good so i don't keep it connected for long. I want to know how much the wire I am using can take. The only thing on the package it came in was what gauge it is, but I thought that related to size not how much it could take. Is there a way to experimentally determine it? Also is connecting batteries in series or parallel better (for getting more voltage)? And finally, for the ideal solenoid B=u0in where i is current, and n is turns per unit length: if voltage through the coil is constant then does just adding more turns get canceled out from the decrease in current due to more wire and hence more resistence so B would remain the same?