A magnetic object can be made into a magnet using the stroke method or the electrical method. When a magnetic object becomes a magnet, we say it has been magnetised. Objects made of iron and steel can be magnetised.
In the stroke method, we stroke an iron nail with one pole of a bar magnet. We repeat the strokes in the same direction each time, always using the same pole. More strokes make a stronger magnet. We test the nail by placing it near steel clips — if it attracts them, it has become a magnet.
We can also make a magnet using the electrical method. We coil a wire around an iron nail, then connect the ends of the wire to a battery. This creates an electromagnet. We test it by placing the nail near steel clips to see if it attracts them.
⚠️ Safety warnings: Do not connect the electromagnet to the battery for too long (it gets hot). Do not touch exposed wires. Do not connect to the main electrical supply. 安全警告：不要将电磁铁接电池太长时间（会变热）。不要触碰裸露的导线。不要接通主电源。
💪 Strengthening an Electromagnet · 增强电磁铁的磁性
We can increase the strength of an electromagnet in two ways: by increasing the number of turns of wire around the iron nail, or by increasing the number of batteries used. More turns or more batteries means the electromagnet can attract more steel clips.
A fair test is an experiment where only one variable (thing) is changed. All other variables must be kept the same. If we change more than one variable, we cannot know which variable caused the result. We should also repeat the experiment a few times to make the results reliable.