Handheld devices are getting smaller, and the memory (and other) add-ons are getting more fiddly. The smallest thing I can easily handle is a tiny button-cell battery, so taking this to the limit of my fingers ...
Make a cylindrical memory device, say, 2mm diameter and height. That should be big enough to pick up, but no bigger. Make a well in the mobile device about 10% larger all round, so the button fits easily into the well and is flush with the surface. Now, build in permanent magnets into the button and the well, so the button easily snaps into place and is safely retained. Ejecting the button is achieved by pulsing a current for a second through a coil built around the well, the magnetic field should be slightly stronger (and opposite polarity) to the magnet built into the well. The magnets also have the advantage that the button cannot be inserted into the well the wrong way round.
Of course, the buttons need not be cylindrical; spherical, hemispherical and other similar shapes should also work (and may even work better than a cylinder). One particular example of a similar shape is a combination of a 1mm radius hemisphere with a 1mm radius, 1mm high cylinder.
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Would the magnetism interfere with the memory mechanism?
...hmmm, "tiddly"--I had to look up that word. You must be british.http://encarta.msn.com/dictionary_1861673867/fiddly.html
They do make micro SD chips, you know. They are 15 x 11 x 1 mm.Anything smaller, and they'd get lost.
Now, thinking on the edge, how about an RFID like memory array, which you don't even have to plug into your PDA, cell phone, or whatever, just hold near it to use it.
Classicsat, I like your idea a lot. Let's say you hava a PDA and you want an electronic add-on. Just open a plain plastic box on the PDA and drop in your grain of sand sized add-on.
The close proximity allows communication. The chips are robust so flopping around in the box isn't much of an issue (foam could even stop that). Now your PDA can communicate and authenticate with all sorts of commercial and custom applications.
For instance going to the grocery store and automatically logging the product and price for everything you walk past. Then when you shop in another store the same function is performed. The software compares the two stores for products and compares prices. You make a shopping list and the software tells you where to buy it for the least money.
Why not just have a drop in container with beveled sides. Then push a button to eject the container. This is already done.
The inertia of a button battery when bumped would more than be enough to dislodge a device held by a magnet. Also, what happens when a little goo from a spilled soda gets on the interfacing surfaces. You'll have to dig out your container.