Coil and magnetic core parts from an electro-mechanical device known as a contactor. Took this one apart to confirm what had happened to it, and take a series of photos to show future classes how these devices work.

Had warned the person that made the board this was fitted to, that using black and red terminals would guarantee that someone would connect DC to the coil rather than the required AC. Even offered him a couple of yellow terminals to match the board he was copying, and also match the yellow AC terminals of the power supply they'd most likely use. "No, I'll just label it up, and that'll be fine." Unfortunately, I was proved right.

Whilst I'm the first to admit that his industrial experience of electrical engineering is vastly superior to this electronic engineer's, but his knowledge of working in education isn't. It lasted about a month and a half, or about five or six class uses, compared to the one he copied, that's still going strong after at least 30 years... since I made it! He'd left before it was broken, so muggins had to replace his contactor with another new one.

Connecting the coil to DC meant that once running, the only thing limiting the current through the coil was its resistance. AC connected to it has a property that we call reactance that combines with its resistance to impede the current flowing - basically, the current is lower.

When someone eventually connected DC to the contactor, it worked, but the coil became rather warm/hot, due to the high current. When they switched it off, the core parts separated correctly - a spring acts to push them apart - but, the heated plastic that the coil was wound/formed around, started to distort as soon as the sprung core section moved out of the coil. The plastic cooled and reset in a different shape. Next time someone tries to use it, the distorted plastic prevents the sprung core from being drawn into the coil, a grumbling like noise is heard, and the contactor's contacts don't make contact... it's broken (polite version).

In repairing the board, yellow terminals were fitted...