Re: How does a curling Iron work?

Date: Mon Apr 24 20:12:09 2000
Posted By: Jim Stana, Mechanical Design/Analysis Manager, Lockheed Martin Orlando
Area of science: Engineering
ID: 955072220.Eg

Message:

I had trouble finding any information on the Web, but I can tell you how they work based on looking at one and my knowledge of design.

A curling iron consists of a heating element, a temperature switch to turn the heating element on and off at a preset temperature, and an on-off switch.

The heating element is just a metal that conducts electricity somewhat poorly. Most metals, like copper or aluminum, conduct electricity rather well and don't get hot unless you are really flowing a lot of current through them. A heating element conducts electricity, but with some significant resistance. Just like the heating element on an electric stove, it gets hot from all of the current flowing through it.

To keep the heating element from getting too hot and scorching your hair, the curling iron has a thermal switch which shuts off the current when the heating element gets to a pre-set temperature. This switch is usually a bi-metallic type. That means it is made up of a disc with two different metals bonded together. Each of these metals expands as they get hotter, but at different rates. Because one expands faster, it tries to get larger than its mate. So what starts out as a flat disc turns into a warped potatoe chip shape. If you have an electrical contact touching the disc, the disc will move away from the contact when it heats up and warps. The circuit is broken (off temperature) and the current stops flowing through the curling iron heating element. As the heating element cools down a few degrees, the disc also cools and begins to straighten out, and eventually remakes the circuit again (reset temperature). When that happens, the heating element starts heating up again. In that manner, the temperature of the curling iron is held within a fairly small temperature range which has been shown to curl hair. (I am not sure what that temperature is.)

If you were to plot the temperature of the curling iron over time, it would look something like the crude graph below.

Off temp                 /\    /\    /\
                        /  \  /  \  /
reset temp             /    \/    \/
                      /
                     /
                    /
                   /
                  /
Main switch on  --
                ----------------------> time

The on-off switch just starts everything going when you want it to and turns it off when your done. Hopefully, you remember to turn it off and don't leave it somewhere where it can burn or melt something like the table you placed it on. That's why the curling iron usually includes a little stand or clip that allows you to place it on a flat surface with the hot end in the air where it can cool naturally to the air by convection before you put it away. The air surrounding the hot element will be warmed and will rise, with cooler air taking its place. As the heating element warms the cooler air, this also rises. Eventually the flow of air will cool the element til it is no longer unsafe to touch.

Since hair is a long string of protein, it acts like a wood fiber. When you heat it and form it into a shape like a curl, it tends to hold that shape (curl) until you wet it. You can make paper curl by rolling it into a tube with your hands in much the same way.

I hope I have helped you.

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