Two in the afternoon.
Shida Affiliated Middle School, Physics Lab One.
It was a simple, even dated, electronics lab.
The windows were half-open.
The air was filled with a unique smell—a mixture of rosin, soldering iron fumes, and old Bakelite.
On six wide, wooden lab benches,
tools had already been laid out.
A bulky, black MF47-model analog multimeter.
Its plastic face was covered in a thin layer of scratches.
Two wires with alligator clips, one red and one black.
A standard 1.5V D-cell battery, its casing wrapped in electrical tape.
In the very center of these tools
sat a square box, sealed shut with black engineering plastic.
The box was about the size of a palm.
There were no seams on any side; it had been completely sealed with strong adhesive.
On top of the box,
four brass binding posts protruded.
They were engraved with four letters: A, B, C, and D.
Professor Wang pulled over a wooden chair with peeling paint
and sat at the front of the lab.
He held an enamel mug in his hands.
He looked like a kindly but cunning old man watching his grandsons play in the mud.
"This morning,"
Professor Wang took a sip of tea and began with a smile.
"All that jumbled nonsense about equivalent circuits and diodes we stuffed into your heads..."
"Now,"
He gestured toward the black boxes on the lab benches with his chin.
"It’s all in this box."
"Inside this box, there are wires and resistors with varying values."
"There’s also a diode, which only conducts in one direction."
Professor Wang set his mug on a tall stool beside him.
"The rules are simple."
"Absolutely no breaking the box open by force."
"You can only use the multimeter and battery on your desk."
"You have half an hour."
"Draw the true internal circuit diagram of the box accurately on a piece of paper for me."
"Which terminal is connected to a resistor, which is connected to the positive end of the diode, and which is connected to the negative end."
"Label everything clearly."
Professor Wang leaned back in his chair.
"Begin."
At his command,
the lab
was instantly filled with the crisp CLICKING sound of multimeter dials being turned.
Wang Huashao snatched the two probes of the multimeter.
He twisted the dial to the ohmmeter setting.
He took a deep breath,
trying to force his still-dazed brain to reboot.
"It’s just measuring resistance, right? Four posts, test them two by two, find the pattern,"
he muttered, giving himself a pep talk.
The red probe’s alligator clip bit down on post A.
The black probe’s alligator clip bit down on post B.
The multimeter’s needle swung quickly, stopping at a reading in the middle of the scale.
"About 50 ohms."
Wang Huashao quickly jotted down on his paper: AB = 50.
Then, keeping the red probe on A, he moved the black probe to post C.
The needle twitched slightly and stopped at an extremely high value.
’The resistance between A and C is huge... Are they in series?’
Wang Huashao’s testing grew faster and faster.
A and D, B and C, C and D.
In less than three minutes, he had completed a full round of tests.
Staring at the six haphazard data points he’d written on his scratch paper,
he tried to piece together a mixed circuit diagram with a diode in his mind.
But,
it was impossible.
He found that no matter how he drew it, the six resistance values led to contradictions.
This was because the box contained not only regular resistors but also a diode with different forward and reverse resistances, and possibly even parallel branches with resistors and the diode.
"Something’s not right..."
Wang Huashao scratched his head, mussing his short hair into a bird’s nest.
He picked up the probes, preparing to re-measure the resistance between A and B.
This time, he casually clipped the black probe to A and the red probe to B.
(The polarity was the complete reverse of his first measurement.)
The multimeter’s needle shot sideways,
slamming all the way to the right.
The resistance was practically zero!
Wang Huashao’s eyes went wide.
He stared intently at the multimeter’s needle, his mouth slightly agape.
’What’s going on?’
’I just measured 50 ohms between A and B. How did it become zero just by swapping the probes?’
He suddenly realized.
"The diode!"
They had just gone over it this morning: forward bias conducts, reverse bias blocks!
This realization brought him no relief.
Instead, it plunged him into even deeper despair.
Because for the six data points he had just recorded, he hadn’t paid any attention to the polarity at all—he’d just been poking around blindly.
How many of them were the equivalent resistance from the diode’s forward bias?
How many were the infinite resistance from its reverse bias?
It was a total mess.
"Brother Kai..."
Wang Huashao turned his head to look at Zhou Kai at the next bench and let out a mournful cry.
"Is this box busted or something?"
"What did I just get for A and C again? My head is still spinning from that resistor cube problem this morning!"
"Why do these stupid values keep changing randomly!"
Zhou Kai wasn’t doing much better.
He was also rubbing his throbbing temples.
Zhou Kai had tried to use a few algebraic equations to reverse-engineer the topology inside the box.
But he discovered that due to the diode’s non-linearity, the coefficients in his equations weren’t constant.
This wasn’t a problem math could solve at all.
"Don’t panic."
Zhou Kai said with a wry smile, probing with the wires in his hand.
"There must be a branch being short-circuited by the diode."
"Take it slow. We have to remember the polarity."
The entire lab
descended into a chaotic bout of trial and error.
The sound of alligator clips clinking against binding posts mingled with the frustrated sighs of the boys.
Human short-term memory, when faced with this kind of multi-variable, patternless feedback—
especially under extreme mental fatigue—
simply crashed.
At the first lab bench,
Lin Yi dragged a chair over.
She wasn’t sweating and prodding randomly like the boys.
She rested her chin on one hand, while the other tapped idly on the desk with the red probe.
The high-intensity theory lesson from the morning had indeed made her a bit drowsy.
She stared at the black box in front of her for a long time.
Then, she reached up and rubbed her eyes.
"How troublesome,"
she muttered.
She pulled her chair forward a little and sat up a bit straighter.
Lin Yi picked up the multimeter’s two probes.
She didn’t test all the combinations.
She glanced at the black box.
’Four terminals. There has to be one main path and a few branches.’
’The diode is hidden in there.’
She switched the multimeter to the DC voltage setting.
She used the 1.5V D-cell battery as an external power source.
She touched the red probe to A and the black probe to C.
The needle deflected slightly.
There was a weak voltage.
Then,
with a nimble flick of her fingers,
she swapped the red and black probes.
Black on A, red on C.
The needle still deflected.
But,
Lin Yi’s eyes narrowed slightly.
The degree of deflection was different from before.
There was a tiny, almost imperceptible difference.
A normal person would have dismissed it as the multimeter’s standard margin of error, or an unstable battery voltage.
But Lin Yi was different.
Her intuition told her.
’This isn’t an error.’
’This is the unique, slight voltage drop from a conducting diode.’
’It’s between A and C, and the positive terminal is on C,’
Lin Yi concluded with certainty in her mind.
She immediately switched the multimeter back to the ohmmeter setting.
She measured between A and C twice.
This confirmed the existence of the voltage drop and also determined the forward and reverse resistance of the branch containing the diode.
Next,
she touched the probes to B and D.
The needle didn’t move at all.
She reversed the polarity.
It still didn’t move.
’B and D is an open circuit.’
It took fewer than four contacts.
Lin Yi tossed the probes onto the desk.
From her faded canvas bag, she pulled out a black gel pen.
She uncapped it.
On her scratch paper,
she swiftly drew a series-parallel mixed circuit diagram that included a diode.
There were no derivations.
No extraneous data tables.
Just the final, complete solution.
CLICK.
Lin Yi capped the pen.
She picked up the paper and stood.
She sauntered over to the front of the classroom.
She slapped the diagram down on the lectern next to Professor Wang.
"Professor Wang, I’m done. Can I rest now?"
Lin Yi rubbed the back of her neck, her tone casual, tinged with a justified sort of exhaustion.
Professor Wang, still holding his mug, glanced down at the paper.
It was perfectly accurate.
He shot Lin Yi a look, said nothing, and simply waved his hand as if shooing a fly.
Lin Yi turned around.
She ambled slowly back to her own lab bench.
She threw her canvas bag onto the desk to use as a pillow.
Then she flopped down comfortably on top of it,
burying her face in the crook of her arm.

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