Top-Secret Top
Background

In 2014, my aunt gave me a top paired with a platform for it to spin on that she had purchased at a garage sale. It wasn’t labeled in any way, but through a bit of online research I determined that it was called a “Top Secret” and was designed to spin for extremely long periods of time until the 9-volt battery that kept the top spinning died. Despite my efforts, I was not able to find any description of the top’s driving mechanism online. Based on the electromagnet and battery inside, I gathered that the electromagnet was spinning the magnetic top in some way, but I didn’t figure out exactly how until a few years later when I revisited the problem and asked my Circuits professor what he thought. He was unable to figure out how it operated when I brought it to him, but when I explained my hypothesis to him, he replied “Maybe”. I was surprised that my professor either didn’t care enough to try and explain it or simply couldn’t understand the mechanism, but I recently tried again to see if there was a full description of the mechanism online and found something that matched my hypothesis.

A YouTube video in which a newer version of the Top Secret top is spun up and runs for about a minute, tagged as an “education” video, was paired with a paragraph description of the top. It described the process through which the top, which holds a radially oriented magnet, approaches a coil under the plastic base on which the top spins and induces a current in the coil, thus triggering a transistor to activate. The transistor allows the battery in the housing to supply power through a solenoid, which acts as an electromagnet, pushing away the top and providing a torque each time that the top moves towards the center to keep it spinning.

This description essentially directly matched my hypothesis, which is confirmation enough for me, since I haven’t been able to find out anything else online. I was curious as to whether or not it would operate properly if I swapped out the 9-volt battery for a 12-volt DC power supply, so I tested that out. It seems that a higher voltage, which can deliver more current through the solenoid, generates a stronger magnetic field and thus larger forces which cause the top to spin more quickly than it would from a lower voltage. Heat doesn’t build up in the solenoid at the low operating voltage, so it could potentially run indefinitely, provided that it has a constant voltage source. I enjoy tearing down mechanisms to determine how they work. It’s fun to reverse-engineer the driving mechanisms, and I almost always learn something that I can apply to other areas of my life.

Detailed Description

As the top approaches the center of the slightly concave plastic base due to gravity pulling it to a lower potential state, its moving magnetic field creates changing magnetic flux through the solenoid beneath the base. This changing flux induces an EMF across the base and emitter leads on a base junction transistor, and when that forward voltage meets or exceeds 0.7 volts (if it's a silicon BJT), a proportional current is allowed to flow from the collector to the emitter. The transistor then allows current to flow through the solenoid, causing it to act as an electromagnet.

The current that the battery in the case supplies through the transistor and copper coil then induces a proportional magnetic field opposing the field of the magnet in the top, generating a force that acts on the top up and away from the center of the base. Due to the concave curvature of the top surface of the base, with sometimes the addition of a bump or line sticking out of the surface, the movement of the top away from the center applies a torque to its shaft via friction, accelerating its angular velocity and allowing it to continue spinning for longer. As the point/tip of the top, which is just a very thin cylinder, rolls past the line or bump on the concave surface, or rides on one edge of the cylinder's end up the concave surface, this torque is generated by the friction between the cylinder edge and the surface that it's dragging past.

A torque is also applied as the top again approaches the center. Examining the top while it operates reveals that its trend of travel around the surface will tend to be counterclockwise if the top is spinning clockwise and clockwise if the top is spinning counterclockwise. This motion relationship is a product of the top essentially "riding on an edge" of its tip, or end cylinder, up and down the curves of the base's surface. Because the net motion in a clockwise-spinning top is counterclockwise, the counterclockwise motion causes the top to ride more on the right-side edge than the left as the top moves up a curved surface, and more on the left-side edge than the right as the top moves down the curved surface when looking at the top from the center of the bowl.

When the entire clockwise-spinning top is being pushed away from the center of the base, since the top is riding on the right edge of the cylinder, there is a slight frictional force pulling that edge towards the center of the base while the rest of the top is pushed away. These opposing forces are the those that drive the torque which angularly accelerates the top. The top may sometimes be found traveling in a tri-lobe or quad-lobe path in the same direction as the rotation of the top, rather than a circular path counter to the rotation of the top. When this occurs, the opposite edge of the top’s end would be geometrically expected to be the one in contact with the base’s surface, which would slow the top with its torque rather than accelerate it.

I struggled with ascertaining what was going on even in my slow-motion videos of the phenomenon. However, I eventually noticed that the top begins its motion on each new curve riding on the edge that applies a slowing torque, as expected, but the top quickly corrects its orientation to ride on the opposite edge, or the one on the “inside” towards the center of the base, which provides an accelerating torque. I expect that it does this because the motion and opposing torque combination is unstable, causing the top to noticeably bounce away from riding on an edge that attempts to slow its motion.

In these two different rotation states, tri-lobe and quad-lobe, the top’s stem tends to point much more towards the center axis of the base rather than remaining vertical as it does in the circular motion state where its motion is the opposite of its spin. Visualizing the cylinder now riding on its inside edge, since the top is tipped towards the center, shows that the geometry at the interface still checks out in these two states and provides an accelerating torque. There are some cases in which the supply voltage is low enough that the current being provided through the solenoid to apply a torque to the top is just barely enough to keep the top spinning, and the top can enter a circular travel state with a very small radius. In this state, the top revolve around the base center in the same direction as it is spinning, and its stem will be tipped towards the center rather than vertically in order to keep the top riding on the correct edge to apply a torque as it precesses around that axis.

Eventually, since the surface is concave, the top tends towards the area of lowest gravitational potential, which is the center of the base. As it is pushed away from the center and up the curved surface by the electromagnet, the top is given more gravitational potential and kinetic energy with which to repeat the cycle. My own Top-Secret top had the line-bumped surface, but I chose to set mine up on the lid of a Pringles can instead after I had sanded the deformities out of its surface because that way, the top spins much more quietly. The system seems to operate just fine but is far less violently on the Pringles lid.

A 9-volt battery works fine to power the solenoid, which is how these Top-Secret tops tend to be designed, but I got tired of replacing its batteries and opted to connect mine to a power supply instead. I now have it wired up to a 12-volt DC input, and it has been running for 6 months without stopping, and before that, it ran for a few collective years in the different places that I’ve lived. It’s a fun toy, and I am happy that I was able to figure out how it worked based on the physics that I have learned online and in school.

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