Ivan Gamble, of Balclutha Primary School, asks :-

How do you make an electromagnet?

John Campbell, a physicist at the University of Canterbury, responded.

For an electromagnet all you require is some insulated copper wire and a battery. Wind the copper wire into a coil. It is best to do this with someone who knows what they are doing because an inexperienced person could ruin the battery or melt the copper wire. When the coil and battery are connected correctly the current loop in the coil produces a magnetic field which will attract other magnetic materials, such as a steel nail or a magnetic compass needle.

Such magnets are used in many devices such as electric motors. The advantage of this type of magnet is that the magnetic field goes away when the electrical current ceases to flow. This type of magnet is often better when the insulated copper wire is tightly wound around a magnetic material such as a steel nail.

One problem is that to get a higher magnetic field one needs either more turns of wire or a larger current. Both heat the wire more (as in an electrical heater), so the magnetic field is limited. There are several ways to overcome this problem. One way is to use only a short burst of electrical current so it doesn't have time to get too hot. One problem is that there is no magnetic field whenever the electrical current isn't flowing. However, the short magnetic field will magnetise a steel needle which is nearby or in the coil. Ernest Rutherford, in his first researches at Canterbury College, used this method as a detector of a short current pulse in an electrical circuit.

Another way is to cool the coil in, say, liquid air to reduce the electrical resistance of the copper wire. The limit to this method is when the large magnetic forces on the wire causes it to flow and mechanically destroy itself.

A more common way these days is to wind the coil using very special materials which have no electrical resistance at all, provided they are cooled to temperatures below at least that at which air liquifes. This is the technique used in Magnetic Resonance Imaging machines. All major hospitals now have these to image the inside of the human body.