So you've decided to hide a cache using a magnet.
There are lots of different types of magnets that can be used to hold your cache in position.
One cheap source of strong magnets is computer hard drives. A couple of examples are shown below. File:Magnets.jpg
The magnets are around the mechanism that works the head assembly. The heads are those little fingers that swing in and out of the platters (just in case you didn't know ). The end of the head assembly furthest from the centre of the discs has a coil of wire and is surrounded by the magnet. Presumably some sort of magneto-motive force is used to move the heads in and out. There is usually one or perhaps two screws holding the magnet in place.
As regards fixing the magnet: some people have had success with putting the magnet on the inside of the cache container and securing it in place with gaffer (duct) tape. The natural inclination of the magnet is to stay attached to the metal on the other side of the plastic, so it only needs to be kept from sliding around; it doesn't need to be fixed solidly. Not much success has been had with attaching things to plastic with adhesives. It's a great glue, but doesn't bond permanently to plastic. Of course, you could use the old trusty nuts and bolts. Hard Disk Drive magnets conveniently come pre-drilled for the purpose. Success has also been achieved using the pre-drilled holes and small pop-rivets. Ensure you fill the hole with silicone for a watertight seal.
Of course you can always remove the metal backing plate from the magnet, however you need to be very careful as the magnet itself is quite brittle.
Some of the methods for removing the backing plate include:
- Using a piece of dental floss and 'see-sawing' it back and foward through the glue.
- Placing the whole assemble in acetone (nail polish remover) which will dissolve the adhesive.
To get to the magnets all you need are some star screwdrivers, available from Jaycar Electronics, DSE or other electronics suppliers.
File:Rare earth rod.jpg File:Rare earth disc.jpg File:Rare earth block.jpgRare Earth - Neodymium Iron Boron magnets are very strong and can be used with great effect. They cannot be drilled or machined and are, like HDD magnets, brittle, so be careful when handling them.
This is a good site to check out their pull value. http://aussiemagnets.com.au/magnets/Rare-Earth-%28Neodymium%29/
e.g. This type has a pull value of 1kg http://aussiemagnets.com.au/product/--6.35-x---5.08mm-Disc-%28Rare-Earth%29.html i.e. In a direct pull it would hold 1kg. Rare earth magnets are pretty pricey though and you need to consider that they will need to exert their magnetic attraction through your eclipse tin or container. The thicker the container, the more pull you need to get.
Success has been had with 6 of these http://aussiemagnets.com.au/product/-10-x---1.5mm-Disc-%28Rare-Earth%29.html attached (glue and gaffer tape) to a 200ml sistema container. You can get them in bulk lots from ebay sellers and http://s.dealextreme.com/search/magnets for around $0.20 each. So you'd be up for about a dollar for enough magnets to attach to a 200ml sistema.
Obviously the larger the magnet, the better the pull, but the higher the cost.
The Rare Earth magnets can be attached to cache containers quite successfully using a number of methods. One geocacher attaches them to the cache container in the following manner:
A small piece of wood with the same thickness as the height of the Rare Earth Rod Magnet.
Drill a hole into the piece of wood the same diameter as the Rod.
Gently tap the magnet into the hole (using another piece of wood as a buffer). Do not hit the magnet with a hammer as it will likely shatter.
Try and leave the magnet slightly proud of the surface to ensure a better grip.
Using silicone coat the back of the piece of wood and attach it to your cache container.
Then from the inside of the cache container screw some small screws into the wood through the bottom of the cache container. This will ensure the piece of wood does not pull away.
Now you can place your cache onto a steel surface with the knowledge that it won't fall off.
If you can stack enough ferrite magnets together you can get them to hold, but their pull is still not terribly strong. The only success I have had with sticking ferrite magnets to anything is to glue them down, then cover them with gaffer tape. The glue is not strong enough to hold he magnet in place and the flexing of the plastic on a sistema container means that quite quickly the glue breaks away. Covering them with gaffer tape ensures that they cannot pull away from the plastic box.
On the bottom of a 200ml sistema container use 6 ferrite magnets like you can get from hobby shops or places like Bunnings in the craft aisle (sometimes).