A few weeks back I bought a heating pad as an inexpensive, energy efficient way to stay warm when reading or laying back on my bed in the evening

A few weeks back I bought a heating pad as an inexpensive, energy efficient way to stay warm when reading or laying back on my bed in the evening. It does well with that — it uses under 50 watts on high, and about 43 watts on low.

One thing that surprised me was the warning on the package that said, “Do Not Plug Into an Inverter”. I thought it that was unusual — isn’t a heating pad a purely restive device? It has unity power factor, so I didn’t couldn’t imagine that there was much active circuitry, just some thin wire that allows enough current to pass and voltage to drop to create warmth. I did a bit of internet research and found that heating pads often contain a thyristor to block alternating current to produce direct current. While you can heat with alternating current, and indeed most electric heaters use AC with a resistive element, I assume the expanding and collapsing magnetic fields aren’t great to have next to your body, especially if you are elderly and frail with a pacemaker or metal bone joint.