# Electromagnet project

I am starting a rather unique project and thought I would ask a few questions before I get too lost. Basically the project is to create an hour glass that can be controlled so the sand will go both down or up. My first thoughts were to use a series of electromagnets in a pulse series to lift the ferric sand up and begin forcing it through the center of the hourglass, while pulling the sand up with another electromagnet at the top of the glass. However, I have very little experience with electromagnets, and have never built one. To make it more complicated it must be able to run on a 110v system. I was hoping I could get some advice on how to create an electromagnet (or series of electromagnets) strong enough to move ferric sand approximately 10 vertical inches. By this I mean size, shape, any special materials (other than the obvious iron and copper wire of course). Also, will I need to make any special modifications to the circuit in order to run on 110v other than resistors? Any help would be appreciated.

-Kenny

## Answers and Replies

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It's a nice idea. I think there are some hurdles ot overcome, the main one I can see is that as you suck the ferric sand with a magnet it will magnetise each particle so you will end up with them all sticking to each other. You may end up with a long thread of this ferric sand going up through the neck.

Staff Emeritus
This looks hard, as the "sand" will itself modify the field. I think you may find this very difficult to control.

I see two problems with this:

1) A uniform magnetic field (without gradient) will not attract a grain of ferric sand. The uniform field will produce a torque if the grain is not round (or if it has a dipole moment).

2) Suppose you need a magnetic field of B=1000 Gauss over a gap of L=10 inches (0.25 meters).

The magnetic field is B = μ0NI/L, or

NI = BL/μ0 = 0.1 Tesla x 0.25 meters/4π x 10-7 = 19,894 amp-turns.

Where (How) will you get this many dc amp turns?

Bob S

This looks hard, as the "sand" will itself modify the field. I think you may find this very difficult to control.

Indeed. Look at the many demonstrations of iron filings and magnets. You could use an electromagnet to hold the magnetic "sand" in place, but I doubt you'll be able to pull it through the hole. With a sequence of electromagnets, you might be able to manage to move clumps of sand through a second, wider pipe back to the top, but a reversible hour glass seems unlikely.

An hour glass that can be turned on and off isn't necessarily a failure, though. You might be able to clump the sand enough to keep it from going through the neck with a magnetic field far weaker than one needed to hold it in place at the end, meaning you could get away with weaker electromagnets. I'd suggest some experimentation with fine iron filings, an hour glass, and some permanent magnets...

Danger
Gold Member
Failbeams, does this thing actually have to run in reverse, or only appear to? The reason I ask is that the latter could be achieved through treachery. I'm thinking of mounting the hourglass inside a "transparent" box with a simple mechanism to flip it upside-down. While it's flipping, the box could be rotated to present a different face with some sort of image-inversion lens in place of the clear plastic.

Find some ferric (ferromagnetic) sand and put it in an hourglass. I have found black magnetic sand (I think magnetite, Fe3O4)) on beaches, and picked it up with permanent magnets. You may have to use a fine-mesh screen (seive) to sift the grain size.

Wrap a coil of insulated wire tightly around the waist of the hourglass.

Pulse current in the wire to stop the flow of magnetic sand through the hourglass.

Turn off the current to let sand flow through the waist.

Build a digital variable duty cycle (pulse width modulated) circuit (NE555?) to control the average flow rate of the sand through the waist.

Bob S

I think you need to forget about the ferric sand and think more about copper or aluminium 'sand'. Use gravity to drop it as normal and an induction arrangement to push it back up.

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thanks for the help everybody! I have been doing a few experiments with ferric sand and it does seem to be too difficult to control. Ill start working with aluminum and copper and see where that goes.