Magnet

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 Araldite. 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.

A geocacher contacted Selleys for their recommended product for poly plastics, and was advised that "none of the Selleys adhesives will adhere to Polypropylene or Polyethylene". Check their individual product data sheets on line if you're not sure. Many of them are also not water resistant in outdoors situations. Selleys actually referred them to Loctite for a suitable product. Loctite 770 Activator/Polyolefin Primer is designed for "difficult to bond substrates which include polyethylene, polypropylene, polytetrafluoroethylene (PTFE) and thermoplastic rubber materials". You can read more about it, and suitable adhesives to go with it, here. Loctite 406 seems to give the strongest bond with polypropylene, which is what most plasic cache containers, such as Sistema, are made from. Loctite The stuff is a bit pricy though.

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.