Engineering an electric blanket

The cord leading to my electric blanket doesn’t heat up. Neither does the 100 or so feet of cable leading to my bedroom from the breaker box. But how are the wires inside the blanket designed to heat up but do so safely? Is it just a matter of tuning the thickness of the wire to get the right cross-sectional area to allow it to heat up yet not burst into flames?

Bonus question: does house wiring in Europe have to have to be double the cross-sectional area of the US due to the higher voltage?

The wire in the electric blanket is “resistance wire.”
Unlike copper, it is designed to have a fair amount of resistance.

Once you use this wire, its a simple matter to figure out the how much power you want to dissipate, and then make the wire long enough to do so.

ETA: Resistance wire is generally Nichrome.

Keep in mind, it’s not just your blanket. It’s also your toaster, electric oven and stove, electric water heater, heating pad (for sore muscles) and most anything else that uses electricity to warm up in your house.

Oh, and light bulbs, same idea, just more resistance.

The heating elements in the blanket have higher resistance to the current. The byproduct of resistance is heat. The connecting wires and house wiring are low resistance, and don’t heat up at normal current levels.
Resistance is inversely proportional to cross-sectional area for a given resistivity. But composition has a great influence on resistivity - so the heating elements in your blanket are not copper, but are probably nichrome (nickel/chromium alloy).

No; it can be half the cross-sectional area. Wire doesn’t care about the voltage its exposed to (this does matter for insulation, but for household wire most of the insulation thickness is for abrasion resistance and the like–only a tiny portion of the thickness is needed to stop the voltage). However, doubling the voltage means half the current (for the same power), and lower current means you can get away with a lower cross-section. From what I understand, a big reason for Europe’s use of higher voltages is because of post-WW2 copper shortages.

i see you have less knowledge of certain other energy forms.

in the electric blanket not only is the wiring of a different material (with higher resistivity) it is much thinner than house wiring, both are part of the heat production.

on older (like many decades) toasters or space heater you can see another type of heating wire, intended to get much hotter than an electric blanket, which are much smaller than house wiring.

Yes, and although they give off prodigious amounts of heat, that’s not their primary purpose. Those are tungsten, which I know from photography.

I will assume that you meant a joke about my username and intended to put a big smiley face after that. :slight_smile:

I do cook with gas, but I bake with electricity. :slight_smile:

The question naturally arises: why do light bulbs use tungsten, while other devices use nichrome?

Nichrome has a much greater resistance than tungsten, which makes it ideal for resistive heaters. A tungsten wire would have to be far longer to achieve the same result; pluc, tungsten is more expensive and harder to work with.

However, in a light bulb you have no choice: it has to run very hot to be reasonably incandescent in the visible spectrum. Nichrome would vaporize at that temperature, and tungsten is pretty much the only metal that would survive. So you need a very long, thin wire, and there is not much space inside a bulb, and so the filament is wound into a nested helix.

That type of filament is commonly referred to as a “coiled coil” or “double helix.” Their primary advantage is not volumetric/spatial efficiency, but better energy efficiency.

You can buy light bulbs with straight wire, single coil, and coiled coil filaments. All else being equal, a coiled coil filament is more efficient than a single coil filament. And a single coil filament is more efficient than a straight wire filament.

Sort of. It’s all related. The primary reason is the reduction of tungsten erosion, since metal that evaporates re-deposits on a nearby wire if it’s tightly coiled like that.

That means you can run the filament at a higher temperature (for a given lifetime), which translates to higher efficiency. If tungsten were lower resistance to begin with, that wouldn’t be necessary–they could just use a short, thick cylinder.

A secondary reason, again related to efficiency, is that a long filament packed into a bulb requires lots of support wires, which cool the filament.

Here’s something interesting : if you want a higher wattage lightbulb filament, or a heating element that emits more heat, you make the element thicker and you decrease the electrical resistance. This causes it to dissipate more power.

Obviously, there is a limit to how far you can do this : eventually, you’d be making your filament have about the same resistance as electrical cord, and then the cord would heat up. Marginal spec electrical cords will heat up in normal use : I’ve noticed that vacuum cleaner cords and electric space heater cords can get noticably warm in normal use.

I was actually referring to the inert gas inside the bulb, and how the gas carries more heat away from a single coil filament verses a coiled coil filament, thus making the latter more energy efficient. Here’s a good write up on it.

From a marketing perspective, “electric blanket” has been mostly usurped by “warming blanket”.

A good article, though it’s not entirely clear how the various benefits break down. It does say:
Although coiled filaments do still lose some heat to the gas by conduction and convection, for the vast majority of lamps the heat loss is reduced so tremendously that the benefits of reduced filament evaporation and hotter operation outweigh the remaining energy loss.

I’m not 100% what the author meant here, though it seems to say that after the first coiling, heat loss to the gas is no longer the dominant efficiency loss.

At any rate, it’s rather amazing how complicated the behavior of a simple light bulb is.