2026年4月28日 / Last updated : 2026年4月28日 桑子 研 Science

The 0.001-Second Standoff: Unmasking the Mystery of Slow-Motion Electricity (Self-Induction Experiment)

I’m Kuwako Ken, your science trainer—where every day is an experiment.

Have you ever heard the word “amanojaku”—someone who instinctively does the opposite of what they’re told? Believe it or not, there’s a component in the world of electricity that behaves just like that. It’s called a coil.

The moment you turn a switch on, it resists: “Don’t let the current flow!”
Turn the switch off, and it clings stubbornly: “Don’t stop the current!”

In this article, we’ll use a modern sensor to uncover the coil’s resistance happening in mere milliseconds—something far too fast for the naked eye to ever catch.

A Contrary Component? The Mystery of Self-Induction

When you include a coil in an electric circuit, the current doesn’t instantly jump when you turn the switch on or off. Instead, it changes gradually. This phenomenon is known as self-induction.

When current suddenly flows through a coil, the magnetic field inside it changes rapidly. The coil “dislikes” this abrupt change and generates a reverse electromotive force to counteract it. It’s almost like inertia in the world of electricity.

You’ll find this in textbooks, but because it happens so quickly, it’s nearly impossible to observe with ordinary ammeters or voltmeters—the needle barely has time to move.

So this time, we used a high-precision sensor, the EasySense V-Hub, to make this fleeting moment visible. A Go Direct current sensor is also a great option.

The Science Recipe: Preparing the Experiment

To capture invisible electrical behavior, you need the right tools.

Things you’ll need
• 3 resistors (20Ω each)
• A coil (0.05H)
• A switch (a push switch works best)
• Breadboard and jumper wires
• 2 AA batteries (3.0V) with a battery holder
• EasySense V-Hub
• Current sensor (100mA or 1A range)

EasySense V-HUB

Let’s Begin! Visualizing Self-Induction

① First, build the circuit as shown below. The key is to insert the EasySense current sensor where you would normally place an ammeter.

We used a breadboard to assemble the circuit. It’s easy to plug and unplug components—an essential tool for experiments.

② Next, configure the EasySense settings so you don’t miss that split-second change.
• Measurement time: 500 ms (0.5 seconds)
• Sampling interval: 2000/500 μs
• Trigger: Start recording when current exceeds 15 mA (with 25% pre-trigger)

This trigger setting is crucial. By capturing moments just before and after the switch is pressed, you can perfectly observe how the current rises.

③ Once everything is ready, press the push switch and record the data.

The Results: A Coil’s Stubborn Personality

Take a look at the graph. The vertical axis shows current (mA), and the horizontal axis shows time.

The blue line represents the case without a coil (just resistors). The current shoots up almost vertically the moment the switch is turned on.

Now compare that to the red line, which shows the circuit with a coil. The current increases gradually, forming a gentle slope. This is clear evidence that the coil generates a reverse electromotive force, resisting sudden changes—essentially saying, “No sudden currents allowed!”

The slight difference in maximum values may come from the coil’s own small resistance (from its copper wire). Encountering these “real-world deviations from theory” is part of what makes experiments so fascinating.

Going Deeper: The Moment You Turn It Off

The story doesn’t end when you turn the switch on. Try placing the current sensor in the position shown in the next circuit diagram.

This time, cut the current by switching it off suddenly. The coil reacts by trying to keep the current flowing in the same direction, almost as if it’s shouting, “Don’t stop!”

This is why you sometimes see sparks when turning off a switch. A coil really hates change.

There are many more fascinating behaviors—like electrical oscillations when combined with capacitors. With sensors acting as your “eyes,” you can explore the hidden drama of electricity that usually goes unseen.

電気が奏でるリズムを捕まえろ！V-Hubで挑む「電気振動」可視化大作戦(V-Hub8)

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