How To Build The Simplest Electric Train

Detailed explanations of how they work and how to build one.


How to Build the Simplest Electric Train:

The world's simplest electric train has no moving parts. No motors. No gears, pistons or actuators. Yet it can race around its track at over five miles per hour, far faster than convention electric trains three times its size and powered 120 VAC transformers. These little miracles that travel with no apparent method of propulsion are homopolar shuttles. They use the same principle of homopolar motors, which are simply direct current motors where current always flows in the same direction, as opposed to most motors whose current reverses, or is commutated, during every revolution. In homopolar motors the magnets that drive them are stationary and the wire rotor moves. In the world's simplest electric train, the wire, the coil the train uses as a track, holds still and the magnets move. Here's how it works:

This cut-away view shows the train inside its track, which is a coil of copper wire. The "train" consists of a battery with a rare earth super magnet at each end. The magnets are oriented so that they want to push each other apart. The battery holds them far enough apart so that their attraction to the metal ends of the battery is stronger than their mutual repulsion. In this example the north ends of both magnets face each other. The train will also work if the south poles face each other, though it'll run through the track in the opposite direction. Electricity flows out of the battery, through the magnet at one end, into the bare copper coil (as indicated by the cyan arrow,) into the magnet at the other end and back into the battery. As the current flows through the coil, it creates a magnetic field, which opposes the magnet in the front of the train (the two green Ns) so it pushes the magnet away. At the rear of the train the opposite happens. The South end of the field attracts the North end of the magnet and pulls the train forward. So the train gets two forces speeding it around the track: the pushing force in the front and the pulling force in the rear.

I have purposely avoided talking about how to figure out from specific field and current directions which way the train will move for two reasons. First: it doesn't matter. Second: doing so is like trying to follow the worst Victorian parlor game ever invented.

Field and current orientations don't matter because knowing them isn't necessary to make this simplest of all electric trains work. Simply hold the magnets so that they are trying to push each other apart and stick them on the ends of the battery. Push the train into the coil and it'll either go or it'll try to push itself back out of the coil. If the second happens, simply rotate the train end-for-end and push it into the track. This time it'll go.

Why is figuring out how current and field directions determine direction of travel so complicated? Here's why:

1. first use a compass to determine the north and south poles of each magnet. That's easy. Then use either the left or right-hand rule to determine the direction of current flow out of the battery. Why left or right hand rule? Because Benjamin Franklin messed up. He thought electrons had a positive charge, which requires a right hand rule to be used to determine magnetic field direction when they move down a wire. By the time everyone discovered he was wrong, so many textbooks had been written and the right hand rule had become so intrenched in technical culture that the experts decided that since it still worked it didn't really matter if physically it was the negative electrons that were what were moving. (Another reason may be that since most people are right handed, they prefer right hand rules.) If you want to use negative electrons as the source of the current, a left hand rule has to be used. Which one are you most likely to prefer? Most physics oriented people are dogmatically supportive of the positive current flow idea. Most engineers, being more realistic (or simplistic) prefer negative charge carriers. Both work. But unfortunately that's not the real stumbling block. That honor goes the the coil.

Depending on how you wind the coil, either clockwise or counter clockwise, and how you look at it, the section of magnetized track that drives the train forward may have a magnetic field pointing either left or right. Which one? You guessed it, another right hand rule. The rule states that you wrap your fingers around the coil in the direction the current is flowing, your thumb will point in the direction of the magnetic field's north pole. But, are you using positive or negative current? Which way is the coil turning? And which way are the magnets oriented? See what I mean?

I managed to figure it out once... then took two aspirin and vowed never to do so again. If you need to do so for a science project it's worth the effort because explaining the intricacies of it all will impress any teacher, but for myself, I'll simply throw the magnets on the ends of the battery, feed it into the coil and live in blissful ignorance as I watch the little train zoom around its track.

Which is not to say I'm completely ignoring the technical side of these simple electric trains. Only that I chose to focus on more practical issues: like how close you should wind the coils, how many magnets should be used and which batteries work the best.


Wind the track's coil too tightly and neighboring coil loops will touch, shorting them out and weakening the magnetic field. Space them slightly farther apart and the train will fly around so fast it's almost impossible to follow. Additionally, closely spaced coils cover up so much of the train that it's hard to see even when standing still. Wind the coil loops too far apart and the train will run painfully slow, will not be able to climb even shallow ramps and will often snag the train because the loops are far enough apart for the leading edge of the front magnetic to catch. To determine which spacing works best I set up the test rig below:


Comparing spacings of 16, 12, 9 and 5 loops per inch to see which threw the train the farthest out of its end showed that the best in terms of trading speed and uphill power for visibility was 9 loops per inch.

Using the same rig and methodology, I found that on a AAA battery with 1/2-inch diameter by 1/8-inch thick N48 rare earth magnets, the best performance came with four magnets on each end. Less and the train didn't slide as far. Increasing the number of magnets to five actually slowed it down. I assume this was because while the extra magnet increased field strength and therefore thrust, its weight increased the drag more.

An interesting point is that because of the limited energy density of batteries, these simple electric trains weren't possible until the invention of high strength neodymium magnets. Only they have enough field strength to drive the train around the track, and even some of them aren't good enough. The first time I tried making one of these trains it didn't work. In time I discovered that the reason was the magnets being used were classed as N35s. That's at the low end of the rare earth magnet strength scale.

N35 - N52 Neodymium magnets

N26 - Samarium Cobalt magnets

N5.4 - Alnico magnets

N3.5 - ceramic magnets

N1 - flexible rubber magnets


In rough terms doubling the N rating doubles the magnet's field strength.


When I switched to N48 magnets the train zipped around the track at very satisfying speeds. N52s would be even better, but are hard to find in the size and shapes required.


Finally, I wanted to determine which brand of battery ran the longest. To do so I stared the train running around a simple circle and timed how long each battery lasted.

Several of each brand and type of battery were tested to establish average performances. Here are the results:

These test should not be used to compare batteries for normal use. The dead-short conditions that exist when the train is running is far removed from the regime for which these batteries were designed.

Why AAA batteries?

I tried AA batteries and while they run faster and longer than AAAs, they are so large they require 5/8-inch diameter magnets so that the train slides on them rather than its insulated side. These larger dimensions limit how tight a curve it can traverse or requires that a larger diameter track (coil) be used, which increases the track's cost considerably.

There's a half-length AAA battery that's used in many videos about these simple electric trains from Japan. I found that they lacked power and had short lifetimes. For me the best overall performance came from AAA batteries, specifically: Energizer Max.


Now that we have our batteries and magnets, it's time to start sticking them together. The negative (flat) end of the battery has a large flat electrode that holds a magnet firmly. The small, buttoned-shaped positive electrode is another thing altogether.

Because it's so small, it's easy for the magnet attached to it to slide off at an angle.


In this position it will quickly hang up in the track. The easiest way to prevent that is to place a brass washer with the positive electrode sticking up through its center hole. The washer prevents the magnet from tilting as much. Steel washers should be avoided because they are attracted to the strongest part of the magnet's field, which is around the outer rim. The washer will prevent the magnet from tilting, but it can slide it so far to one side that it's almost as bad.

For more secure options, consider the following:


Starting on the left. The first option uses epoxy to glue a small brass washer to the center of the magnet. The hole should be reamed out so that the positive electrode fits into it snuggly. If the washer is thick, it may need to be slightly countersunk because the base the the positive electrode isn't perfectly flat. Even better, make a custom steel washer out of a piece of 0.040-inch thick metal strapping. This is shown in the example second from the left. Again, use epoxy to secure the washer to the center. Then, sand the washer down to around 0.038-inches thick (for Energizer Max batteries) so that when the magnet is on the battery all surfaces are flush. The problem with this technique is that it requires the use of an exacto knife to cut off the insulating ring of plastic around the outer face of the positive end of the battery. If this isn't done then the steel washer won't make contact with the metal end of the battery and may not be able to conduct electricity. This is the most secure method and the one I use... half of the time. The other half I buy 1/2-inch diameter, 1/8-inch thick N48 rare earth magnets with countersunk holes in their centers. It turns out the hole is an almost perfect fit for the button electrode. The third example from the left shows one of these magnets with the hole facing up and the countersunk bevel facing down. This technique also requires cutting away the insulated end of the battery and benefits from a slight additional countersink around the small hole to compensate for the battery's uneven face. Turning the magnet over to use the existing countersink doesn't work well because the countersink is so large it allows the magnet to slide around. I do not recommend trying to grind a hole in the face of the magnet to receive the button electrode on the positive end of the battery. (Far right.) I tried this once and quickly gave up. Besides the question of rare earth toxicity and the grinding process throwing magnet chips into your face, the materiel of the magnet almost seems to burn. Sparks linger much longer than they should and it's common to see persistently glowing flecks on the magnet's face. It's almost as if the magnet's material is flammable.

Once I'd determined that a stack of four, 1/8-inch magnets worked the best, I substituted a single 1/2-inch thick magnet for the stack and discovered that it ran around 20-percent faster.


A single magnet has the advantage over the stack because the individual magnets in a stack tend to slide over each other. Even if the base magnet is centered on the battery, the magnet at the other end can be off to one side far enough to catch on the track.


Batteries often have exterior patterns designed to catch the eye so that they are purchased. Inside a coil, these often irregular patterns act like camouflage making it difficult to see the train. In the following image...

... the battery has been covered with white electrical tape and the track placed on a dark cloth to make the train easier to watch. I've tried black trains on a white cloth but they aren't as easy to see.


I wind my tracks (coils) on 1/2-inch electrical conduit. One way to do so is to fix a drill bit that fits snuggly into the tube and then chuck the bit into an electric drill. While a helper controls the drill's speed, wire is pulled off a free running spool. To get an even coil with 9 loops per inch, I use a length of standard bamboo skewer as a spacing guide. The loops will look too far apart during winding, but when the ends are cut loose the coil springs closer. The best wire is 18 gauge soft, bare copper wire. Prices average $17 - $24 a pound (199 feet) on One such spool will be enough for 12 feet of track.

A coil about to be wound. The skewer will be placed between the last loop and the wire coming off the spool during winding.


Once a length of track is wound, it must be treated with care because the copper is so soft that its own weight is enough to stretch it out of shape.

If I need to lift it, I hold it in several evenly spaced locations to distribute the load.


Long tracks have to be made by connecting short lengths of track. When doing so make sure there is good electrical contact between the two sections and that there are no bumps that could stop the train. I use two connection techniques.

The smoothest and most secure way is to wrap the union with clear boxing tape. Fold the outside end over to make a tab for easy removal. If the track is being used for a demonstration where different lengths need to be connected and disconnected quickly, clear plastic hair clicks work well. They can be improved by cutting off the last 1/4-inch of the tines and covering them with clear tape so that the tines don't poke through the side of the track and stop the train.


Adding ramps is a great way to increase the entertainment value of a track because it enables one track to go up and over another. The question is: How sharp an angle can one of these trains manage?

Using four N48 magnets at each end of a new AAA battery, this test rig was set at a variety of inclines. What it indicated was that while such a train running in a 9 turns-per-inch track can climb an incline with a rise of three inches over a run of 12, reducing the incline to 1 inch in 12 proved more reliable, particularly if the inclined track also has a curve.


And speaking of curves, the same train running along a spiral jammed when the curvature was less than 3-inches in radius. In actual setups, I find using curves no tighter than 6-inches in radius works better.


Many people leave the ends of their tracks open so that the train makes a single pass. Others like to connect the ends so the the train keeps running around and around until the battery wears out. Here's an example:

The problem is getting the train started. Usually this involves pushing it one end of the track, then rushing to close the two open ends with tape or a clip before the train gets all the way around the track. It's awkward and cumbersome. Fortunately there's a more elegant way to start one.


Meet the open coil section:

It's a short gap in the track where the train rides on top of two tightly wound coils. How it works is that while most of the magnetic field is confined inside the coil, enough leaks out around the outside of the coil to keep the train moving forward. There's just one trick to making this work: if the track's coils are wound one direction, the coils for the open section have to wound in the opposite direction. The reason is that the direction of the magnetic field outside the coil is the opposite of that inside the coil. By winding the open section's coils in the opposite direction of the main track, the magnetic field on the outside of the open section will be in the direction as the magnetic field inside the track so the train will keep moving forward. Equally important, the track coils have to be in good electrical contact with the open section coils and the transitions from one to the other must be perfectly smooth.

This is one time where AA batteries have a distinct advantage. They pump out so much current that they have enough thrust to bounce over small bumps. AAA batteries have to have things very flat for this to work. A light coat of graphite helps keep the train moving over this section. The train runs much slower over the open section than inside the track. The coils should also be wound tighter so that the train slides over them as smoothly as possible.


As the train runs around the track, its weight can move the track around. Some people like this. Others prefer a more rigid construction. One way to hold the track in place is to attach small pieces of epoxy putty to bits of poster board to act as cradles to hold the track. Triangle section balsa also works well for the sides. A little double faced tape on the bottom helps them to stick in place on the surface on which the track rests. I like painting mine to make them as inconspicuous as possible.


One final piece of equipment that's nice to have is a starter gate. This is a length of thin insulating material, card stock in the example below, that prevents one end of the train from making contact with the track until the lever is rotated.


There's no limit to how long or complicated a track can be made. The following example has 40 feet of track, five inclines, one passover and a spiral:

To make it even more interesting, various moving features can be added. These all work the same way. A piece of steel at one end is caused to move by the magnetic field of a passing train. Where rotation requires counter balancing, a non-magnetic material such as copper is used. It's even possible to use magnetic switch to cause lights to flash.


These simplest electric trains are fun and interesting to build and use. I hope this page inspires you to give them a try. If you'd like to view a live action version of this page and see the train in action, please click on the following YouTube video:



This second videos provides hints for increasing the performance of the world's simplest electric train:


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