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Physics and Science of Manufactured Goods: A Bit of “Inductive” Reasoning

Chocolate-induction-cooking-recipesEver set your toothbrush on it’s charger and wonder how it charges? It’s just plastic on plastic with no metal contacts. Ever see an induction cooktop work? You can place a paper towel between the cooking surface and the pot and the paper never burns, only the pan gets hot. What about the mechanical hard drive in your computer? You may be surprised to learn there is no direct contact with the storage medium either. So what’s going on here?

The application of magnetic induction principals in design and manufacturing is fascinating and quite clever actually. That you can build such versatile products and elicit a desired effect by simply taking advantage of the laws of physics, without any direct physical connections!

With just a little bit of scientific theory you can open up so much more of your world, small things will start to jump out at you that you may have overlooked before…this is where it gets fun!

Magnetic Induction: Conceptually Speaking

To get at the heart of it we need just a little physics, then we can see how it’s applied to produce commercial products we use everyday. There’s no final afterwards so no groaning.

Michael Faraday discovered electromagnetic induction in 1831, although there were other observations of the phenomena and concurrent discoveries being made as well. Defined by Wikipedia, “Electromagnetic induction is the production of an electromotive force (emf), or voltage, across a conductor when it is exposed to a time varying magnetic field.” Eye roll I know but keep reading.

The law works in one of two symmetrical ways: either by a change in a magnetic field (our “time varying” variable), or by an alternating current (AC). When there is a change in the magnetic environment (by rotating or moving a magnet) within a coil of wire, there will be an “induced” voltage (emf) across the coil. Note that alternating current propagates by changing direction periodically, so reciprocally when there is an alternating current in a wire then a continuous pole reversing magnetic field is created around that wire.

The level of voltage output can be manipulated by either increasing or decreasing the number of turns of the wire in the coil (see “how transformers work”), or by the rate at which a magnetic field changes. The rate itself can be changed by how fast a magnet moves or by changing the strength of a generated field.

Hopefully you’re starting to see the big picture here; A magnetic field can induce a current in a coil of wire around it, and a current can induce (or create) a magnetic field around that wire. Again this is accomplished without any direct physical connections. There is of course a lot more to it, lots of math and theory (see James Clerk Maxwell) but this is the basic point to get across. If you’re confused keep reading, the product examples should make it clear enough.

OK so how does all this help me brush my teeth, store photos, or hard boil eggs?

Electric Toothbrush Charging

An electric toothbrush is an example of an assembly, a collection of manufactured parts (or sub-assemblies) that are designed and arranged to produce a desired effect. The plastic housing is created by injection molding, the motors are specified and sourced from a specialty company, there are steel rods, bearings, a silicon on/off switch (created by overmolding one previously molded part within another mold), gears, a printed circuit board assembly, and of course rechargeable batteries. Every part has a 2D drawing with material specifications, 3D CAD files, there is a final assembly drawing that shows the construction, and lastly a bill of materials or BOM. The designer should also specify and provide instructions by which the units are to be subsequently tested.

The internal batteries themselves are charged by magnetic induction. Within the base charger there is a coil of wire that is energized by the alternating current (AC) from the wall socket, creating a changing magnetic field around that coil. Within the toothbrush handle is another coil of wire. When you place the toothbrush on its stand the magnetic field of the charger is close enough to “induce” a current in the wire coil of the toothbrush. This current ultimately charges the batteries. Pretty neat, huh?

Induction Cooktop Stoves

Induction cooktops is also an assembly, although a bit more complex. What’s really interesting about these devices is that the cooktop surface does not heat up to cook your food, your pan actually becomes the heating element. Below the stove surface you have a wire coil that is again energized by the household AC current. This alternating current therefore creates an alternating magnetic field around it. When you place the pan on the surface the pan itself becomes that second wire coil (like in our toothbrush) and experiences an induction of swirling (eddy) currents within. Because the pan itself is not a good conductor, the induced current experiences resistance which generates heat, ultimately cooking your food. See the link at the end of this article for a fun demonstration video.

Mechanical Hard Drives

Hard drives are manufactured and assembled in China. Again it’s important to note how parts can come together from different manufacturers to assemble a final product. Take the housing itself, if made of plastic it’s an injection molded part. It also contains motors, connectors, metal fabricated parts, and a logic or printed circuit board (PCB). This is complex to explain briefly but a mechanical hard drive also used magnetic induction to write data.

The storage medium of a hard drive is a platter of aluminum, glass, or ceramic…as long as it can’t be magnetized as it would interfere with the operation as we shall see. The platter itself is coated with several thin layers of metal with the top layer consisting of tiny particles of a cobalt oxide.

The read/write arm of the hard drive is positioned as little as 3 nanometers above the surface of the disk (one nanometer is a billionth of a meter). The end of the write assembly has evolved but basically it contains a small metal component with a finely wound wire. Your computer writes and reads in binary code so when it wants to write a “1” it would send a current through the wire in one of two directions. The current through the wire creates a magnetic field which magnetizes a particle directly underneath in either a North-South (N-S) or South-North (S-N) orientation. If the computer wants to write a “0” then the current is reversed, which in turn reverses the induced magnetic poles around the wire, magnetizing the particle in the opposite direction. To read back the data the arm passes over the disk and a read sensor senses the magnetic field of the surface particles. The now magnetized particle will be read according to it’s magnetic field properties, whether it is aligned in a N-S or S-N orientation.

Your computer of course does this incredibly fast, with some disks spinning at 7200 or 10,000 RPM. In binary a string of bits represent data, for example if you want to store the word “cat” the corresponding binary code would be “01100011 01100001 01110100”, with each of the 1’s and 0’s being a magnetic particle in a N-S or S-N orientation.

There are also dual sides and stacked disks in one drive to increase the storage capacity, as well as a sophisticated organizational system of data into tracks and sectors. This just touches the surface of hard drive technology but if you’re interested in learning more there is a lot out there to educate you.

Other interesting applications include the magnetic strip on the back of your credit cards..that’s why you “swipe” it, to create a moving magnetic field, inductive lighting designs, motors, charging pads, transformers of course, and current sensors.

Natlus URD works with all types of assemblies and components, including those products that operate on the principal of magnetic induction and otherwise. Please contact us to submit your designs or request additional information about how we can break down your designs into basic science and work with you to manufacture quality products. See us on the web at

Induction cooktop demo:

How hard drives are assembled:


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