Electromagnetic Ring Launcher -- Building a Science Museum Classic

The Jumping Ring or Electromagnetic Ring Launcher experiment is a staple of science museums and physics class rooms. Here's how to build your own right in your home.

There's a very common demonstration in  science museums and physics-classrooms called the “Jumping Ring” or “Electromagnetic Ring Launcher." The experiment involves a several centimeters long cylindrical iron core inserted in a large solenoid and a copper ring runs through the extended iron core. When the solenoid is powered by the AC mains, the ring jumps out of the core.

There are various reasons for the experiment being so popular and so significant in science and engineering. Firstly, it is interesting to observe a metal ring jumps out or hover. Secondly, it utilizes Faraday’s law of induction, Lenz’s law, mutual inductance, and forces due to electromagnetic induction to make it possible for the ring to hover or jump. The main problem with this type of conventional ring launcher is its bulky size and weight, as it requires a large number of turns of thick copper wire for the solenoid and heavy iron core inside. Furthermore, as it operates at the mains-line voltage (115 V or 230 V, AC),it is not safe to operate. Calculations show that the launching of the ring is many times more efficient at frequencies several times higher than that of the AC mains (50/60Hz).

In this project I used a square wave generator of adjustable frequency from 700 Hz to 18 kHz employing a 555 timer IC; the output of which drives a power MOSFET. The MOSFET drives a small coil of ~50 – 60 turns wound on a 10 cm long ferrite cylinder instead of an iron core. A copper ring is placed through the extended part of the ferrite cylinder. A 16 micro Farad film-capacitor is placed in parallel with the coil to achieve parallel resonance. At resonance the current through the coil can be achieved several times higher than that is supplied by the power source. Using a thick copper wire (AWG #14) for making the coil, the coil-resistance is lowered, which makes the quality factor (Q) of the coil high. High Q of the coil maintains nearly ~ 8 times higher current than the power supply can provide. The high primary current is essential to induce high current in the coper ring, the interacting field makes the ring levitate. The circuit needs only 24V DC do levitate, hover and shoot the ring. A 10-Ohm resistor is used in series with the 24V power supply, as the frequency of the oscillator is slowly increased, supply current goes down gradually. At resonant frequency supply current reaches to minimum, (~1.2 A), and also at this point the copper ring levitates and hover midway on the extended ferrite rod. Another switch is used to short the 10-Ohm resistor, when it is shorted, the ring jumps few centimeters out of the rod. Now, keeping the 10-Ohm resistor shorted, if the power supply is turned ON, the ring jumps tens of centimeters above the rod. This video shows these effects.


Full build instructions and parts list continue below: 

The circuit:

The circuit consists of a square wave oscillator implemented by a 555 timer IC, a power MOSFET and a MOSFET



Note that with a 15 Volt supply to the 555 and R1 at 180 Ohms, you'll have over 80 milliamps running through R1 into the discharge pin during the discharge cycle. The discharge transistor may well come out of saturation at that current, resulting in substantial power across that transistor. You run the risk of destroying your 555. The ST datasheet for the NE555N shows a worst-case saturation voltage of 480 mV at only 15 mA when operating at Vcc=15V. You have more than five times that current.

Hello William: I think your information is wrong. If you take a closer look at the data sheet of NE555, you will see that at Vcc=15V, Idisch=15 mA, Vsat has a minimum to a maximum value of 480 mV. Furthermore, take a closer look at the footnote, even if you take a lower value for R1, there is an internal current limit. At 80 mA, the collector emitter voltage Vce of the discharge transistor is less than a volt. Now considering the duty cycle calculate the power dissipation and consider the therm

thermal resistance Rthja. you will find the package (DIP8) temperature is well below the maximum allowable range. Practically, I ran the circuit for quite a long time, without overheating or damaging the IC NE555

Does the circuit being resonant limit the rate of current increase in the coil? I've only ever made a launcher (not for hovering) and needed to get the current in the coil to maximum before the ring left the field. I dumped about 30A @ 240V into the coil. My launcher gets the ring 2m to 3m into the air at room temperature. When the (aluminium) ring is cooled in liquid nitrogen then it reaches the top of a three storey atrium - about 10m. It's used for university demonstrations.

Hello John: Nice to know your wonderful experience. The detailed analysis of a similar circuit is published in the journal "Physics Education" Jan., 2016. The use of resonance gives us the advantage that if the supply current is ~2A, actual coil current is ~8 to ~ 10 times. So, we need a smaller DC supply. Furthermore, at higher frequency, the upward force is also higher, as it gives larger phase difference between induced voltage and current in the ring

Wow! I built one of these in 7th grade. I must have been about 13. The core was made of laminations of black stove pipe iron: sheet metal strips varnished so they wouldn't conduct eddy currents. The strips were 1" x 12", laminated into a core 1" x 1" x 12". The ring was a stack of aluminum sheets, about 1.5" ID and 4" OD. The coil was a few hundred turns of AWG 18 or 20 wound between two 1/8" masonite spacers maybe 3" apart on one end of the core. Ring never hovered but did fly off impressively

Hello Laurence, It is nice to know your experience. This jumping ring effect really makes lot of kids curious, even today. I was also fascinated when I was a kid, but never tried anything at that time like you. Thanks for your comments.

Can you post more info on making L1? The photos show a toroidal coil with about 25 turns on it, but the text says 40 turns on a cylindrical core - at least that's what I get out of it.

Hi Mark: I wrote the instruction for an air core coil, it will work, Though, in the photo, you can see I used a toroidal ferrite core with few turns on it. I do not recommend doing it, because most of the toroidal core you buy have high permeability and high hysteresis loss at the frequency of our interest. I used the core, as I have it at my disposal which has a low permeability and low loss. However finding a similar one in the supplier's stock is a time consuming job. Rather, it is better to

make one with air core. Though the number turns are larger, it will not affect the normal operation. If you have a ferrite rod, you can also make the coil with 10-15 turns on it. this is also better than choosing a ferrite toroid. The purpose of the coil is only to prevent transient high current. I have a pretty bad experience with various types of toroids. Most of these heat up and hamper the normal operation by consuming extra power.


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