This lecture covers the step-by-step construction of an electromagnet, starting with marking the center of a bolt, wrapping a wire around it, fixing the electromagnet on a ruler, adding a neodymium magnet, attaching a battery, and testing the electromagnet with paperclips. The instructor emphasizes the importance of using alkaline batteries to ensure proper contact and warns against using zinc-carbon batteries due to their non-magnetic ends, which can lead to overheating and rapid discharge.