penny sorting

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This article shows how I sort pennies by their copper and zinc content using magnets. Pennies made before 1982 are mostly copper. Pennies made after 1982 are mostly zinc. Pennies made on 1982 may be either mostly copper or mostly zinc.

Video Demonstration

Copper and zinc are not attracted by magnets because they are not ferromagnetic; however, they will experience a force on them when they move through a magnetic field. Both copper and zinc are electrically conductive, so a counter-electromotive force will be induced in them when they move through a magnetic field. The cause of this CEMF is due to 'eddy currents'. That is, CEMF is a force caused by eddy currents as explained by Lenz's Law.

The practical upshot of all this is that if you shoot a conductive object through a magnetic field it will slow down. The amount the object slows down depends partly on how conductive it is. The more conductive the object the more it slows down. Copper is much more conductive than zinc. If you shoot a copper penny through a magnetic field it will slow down significantly faster than a zinc penny.

I built a simple apparatus that demonstrates this effect by sorting pennies into copper and zinc. In my experiment I sent a total of 231 pennies through the apparatus and sorted them with the following results:

	           | Copper  | Zinc  | Total
	-----------+---------+-------+--------
	 good sort |     50  |  174  |  231
	 bad sort  |      4  |    3  |    7
	 % good    |     93% |   98% |   97%

The amount an object slows down also depends on a number of other factors, of which the most important is the strength of the magnetic field. In my experiment I used an array of neodymium magnets. These powerful permanent magnets can be found in ordinary hardware stores.

In my experiment I glued an array of magnets to the outside of an aluminum channel. I used a 3 foot length of 3/4" channel. The dimension refers to the inside width of the channel. This type of channel is used to frame the edge of 3/4" plywood. It is often sold as 3/4" aluminum channel for plywood. Pennies are 3/4" in diameter, so they perfectly fit this type of channel. The channel is set so that the profile resembles a C instead of a U. The pennies should roll down so that their faces barely touches the channel. Most of the contact should be with the edge. I needed to angle the channel slightly to ensure a tiny bit of contact between the penny face and the channel; otherwise, the pennies would sometimes roll out of the channel. This is an important variable in tuning the apparatus.

I believe there are actually two CEMF forces acting on the pennies. There are linear as well as rotational CEMF forces. The penny obviously moves in a straight line though the magnetic field, but it also rotates thought the field because the penny rolls down the channel. The CEMF due to rotation acts as a break only if the edge of the penny has a friction surface to act against; otherwise, slowing the rotation of the penny has no effect on its linear speed. In this case the aluminum channel acts as a breaking surface. I have not tested how much this breaking effect may contribute to the total breaking effect.

If the rotational breaking effect is significant then there may be an additional set of variables to keep track of. As the device is used the aluminum may start to become polished and smooth. Dirt and oil in the channel or on the penny will also effect the friction. If the channel becomes smooth or lubricated the breaking force will be significantly reduced. I have not tested the difference between a new channel and a polished or lubricated channel, so this difference may be insignificant or nonexistent.

The CEMF due to rotation may help to explain the cause of the occasional error in sorting. As the penny rolls down the channel it may have an irregular edge or it may encounter dust or grit which causes it to bounce slightly and loose contact with the aluminum channel. If it looses contact with the aluminum when the eddy current forces would be causing the maximum breaking then the penny will not slow down as much. In theory this would cause a copper penny to exit the channel too fast and overshoot into the zinc hopper. You would expect that this would cause the zinc hopper to have a greater number of errors than the copper hopper. In practice this was not the case; however, my experimental setup was ideal for capturing this data. A number of pennies also overshot the zinc hopper and landed on the ground. I did not count these in my data; although, I noticed that both zinc and copper pennies were present in this group. I should have counted these. These should be tracked in future experiments. Only about 5 or 10 pennies landed on the ground. I picked these pennies up and fed them back into the device where they were sorted normally.

The main limitation of this technique is the speed at which you can sort pennies. If you drop a copper penny down the shoot closely followed by a zinc penny there is a chance the zinc penny will bump into the copper penny. One way around this problem is to use an alternating magnetic field. An AC magnetic field will repel a conductor. If you attach a coil to a 60 Hz AC power supply the coil will push the conductor away from it. I have used this effect to levitate aluminum plates. I have not attempted this with pennies yet, but the idea is to simply drop pennies from a hopper above an AC electromagnet. As the pennies fall through the AC magnetic field they should be kicked away. Copper pennies would be pushed farther away than zinc pennies. A fence below the electromagnet can be used to direct copper and zinc pennies into different hoppers.