Magnetic/Non-Magnetic

[gillian17]

I have just seen a heavy block of metal, possibly iron or steel, it was about 1.5" cube with a threaded hole in the bottom that looked as though it could fit on a tripod.
The extraordinary thing was it had a piece of steel running through it and protruding either one side or the other like a bed side lamp switch.
When this was pressed in at one side the block became a powerful magnet, press it from the other side it lost its magnetism.
There was a band of another metal, possibly brass, which looked like it passed all through the block, and I was told this but could not see it, a similar though much thinner band near the other side of the block.
It is not an electro-magnetic unless it works on the pd between the two metals but it seemed much to powerful for that.
Beats me.

[Rheghead]

I have used one of these before in my engineering days. It is a magnetic base for attaching a dial test indicator. Don't ask me how it works, some things are best left to marvel at! :)

[George Brims]

I've used those a lot too, for attaching optical mounts (lens holders etc) to a steel or cast iron optical bench. I always wondered how they work. I suppose there's some piece of non-magnetic material inside that blocks the field when the button is pushed one way. I will ask around and see if anyone here knows the answer. One of the profs I work for still has that boyish curiosity about the world that's been beaten out of most of us. If I ask him he is liable to take one to bits!

OK I found an explanation for the operation of the kind that have a knob to turn instead of a push button; it's on Wikipedia at http://en.wikipedia.org/wiki/Magnetic_base. The push-button type presumably work on a similar principle, just with a different geometry.

[Dusty]

I'm going to look really clever tonight at the pub when I explain the operation of the switchable magnetic base after having craftily introduced it into the conversation.
Now....football to magnetism....Hmmm.

Thanks for finding that for us George, I have to confess to having wondered myself over the years how they worked.

[gillian17]

Thanks for that.
I am still not sure I understand it even after the wikipedia entry, but I do remember a faint circle on one (at least) side of the cube. Must admit I did not think to look if it was turning.
I will have to look again in the morning.
Cheers