1. Ampere-turn – the product of the number of turns of wire multiplied by the amperes is called ampere turn of the coil ampere-turns is a measurement of the magnetic field strength of the coil. 2. Electromagnet - a soft metal core made into a magnet by the passage of electric current through a coil surrounding it. 3. Induction – if a second nail is brought close to the end of the first nail it will also be attracted as this is magnetism by induction which is the basis of the theory of magnetism. 4. Lodestone – this device was the earliest compass the magnetite stones became known as leading stones or lodestones.