Chapter 52: Magnetism and the Soldering Iron

05:30. Eyes open. No alarm clock. Body clock.

The ceiling gray-white. Where the wall paint had peeled, a dark yellow layer of putty showed through. Outside the window, dawn had not broken. Wind passed through the crack in the window. Gave a low whistle.

Lin Chen sat up. Moved lightly. Did not shake the bedboards. First he felt the sole of his foot. The gauze was dry. A scab had formed along the edges of the crack. He pressed it. Slight pain. No seepage. He took a cotton swab. Dipped it in mercurochrome. Applied it lightly along the edge of the crack. The medicine evaporated. Brought a brief coolness. Not pungent.

He dressed. Old sweater. Frayed collar. Pilled. The zipper on the outer jacket snagged. He let one drop of sewing-machine oil into it. Pulled it back and forth three times. Smooth.

Cold water. Piercing. He scrubbed his face. The towel was rough. Rubbed his skin. Left red marks. Awake.

He spread open his electromagnetism notes. Pencil moving.

Key topics: Ampere force. Lorentz force. Electromagnetic induction. Alternating current. Difficult points: Dynamic circuit analysis. Transient processes in capacitive circuits. Strategy: Write the equations first. Algebra after. Check dimensions. Check boundary values.

The pencil tip paused. He closed the notebook. Stuffed it into the canvas bag. Pulled the zipper shut.

05:50. Corridor. Sparse footsteps. Someone coughing. Someone turning pages. Chalk dust and the smell of old wood floated in the air. Cold.

Lin Chen walked to the door of the academic affairs office. Dim yellow light showed through the glass window. Old Li sat behind his desk. A red pen in hand. A stack of graph paper in front of him.

Lin Chen knocked. Three times. Lightly.

“Come in.”

He pushed the door open. Heat hit his face. The temperature rose sharply. A faint sting in his nostrils. He handed over the graph paper.

Old Li took it. His gaze swept across it. He did not raise his head. The red pen moved over the paper. Rustle, rustle.

“For the slope calculation. Keep three significant figures. You wrote four. Minus one point.” Old Li stopped writing. Looked up. His gaze calm. “When the load current is 0.8A, the voltage drop is 0.04V. You calculated 0.042V. Instrument precision is 0.01V. The extra 0.002V is noise. Not data.”

Lin Chen nodded. “Noted.”

“The curve is smooth. No break points. Pass. Leave it on the desk.” Old Li pulled out a fresh sheet and pushed it over. “Practical this afternoon. Soldering. Bring your own tools. Rules unchanged. Anyone without them may borrow. Deposit deducted. Limit two times per day. Overtime charged by the hour. Also. Any cold joint. Direct zero. No retest.”

Lin Chen took it. The edge of the paper was sharp. Dug into his palm.

“Understood.”

He turned. Went out. Cold wind poured in. Big temperature difference. Gooseflesh rose on his skin. Breathing steady.

06:00. Bell. Short. Shrill. Cut through the cold wind.

Exam room. Tiered classroom. Long desks. One meter between them. Deep scratches in the tabletops. Grain exposed. On the blackboard: Electromagnetism Written Exam. Time limit 100 minutes. Full score 100.

Lin Chen sat in the third row by the window. Fixed seat. Centered. Neither to one side nor the other.

Canvas bag by his feet. He took out his stationery. Two sharpened pencils. One eraser. One ruler. Three draft notebooks. Arranged in order. No crossing. No overlap.

The exam papers were handed out. Rough paper. Strong smell of ink.

He turned the page. Read the questions. Did not rush to write. Scanned the whole paper first. Distribution of question types. Multiple choice. Fill-in-the-blank. Calculations. Final problem. Gradient of difficulty. Allocation of time.

Multiple choice. Fifteen minutes. Fill-in-the-blank. Twenty minutes. Calculations. Forty-five minutes. Final problem. Twenty minutes. Margin. Ten minutes.

The pencil tip came down. First question. Direction of Ampere force. Left-hand rule. Direct choice.

Second question. Lorentz force does no work. Concept question. Direct choice.

Third question. Capacitive circuit. The instant the switch opens. Capacitor voltage unchanged. Write the KVL equation. Solve for the current. Choose.

The pencil tip did not stop. No hesitation. No looking back. When he hit a snag. Circle it. Mark it. Skip it. Do not get stuck.

Time passed. Only the rustle of pencil on paper. And the occasional crisp sound of pages turning. Someone sighed. Someone spun a pen. Someone cursed under his breath.

Lin Chen did not raise his head. Fingers steady. The draft paper used by sections. Left side for formulas. Right side for calculations. Blank space in the middle. For verification.

Third calculation problem. Electromagnetic induction. A metal rod cutting through a magnetic field. Rail resistance neglected. The rod has resistance. Find the terminal velocity.

He wrote the force analysis first. Gravity. Support force. Ampere force. Friction. Wrote Newton’s second law. F_net = ma.

Ampere force formula. F = BIL. I = E/R. E = BLv. Substitute. Get F = B²L²v/R.

Equation closed. Solve for v. Substitute the numbers. Dimensional check. Meters per second. Correct.

Boundary-value check. If B approaches zero, v approaches infinity. Fits physical intuition. If R approaches zero, v approaches zero. Fits short-circuit logic.

He wrote the answer. Units. Significant figures. Checked it. No error.

The pencil tip paused. He turned to the final problem. Alternating current. Ideal transformer. Ratio of turns in the primary and secondary coils. Load variation. Find the maximum output power.

He frowned. Did not panic. Drew the equivalent circuit. Referred the secondary load to the primary side. Obtained equivalent resistance R' = (n1/n2)²R.

Source internal resistance r. Output power P = I²R'. I = E/(r + R'). Substitute. Get P = E²R'/(r + R')².

Take the derivative. Or complete the square. Get the result: when R' = r, P is maximal.

He wrote down the derivation. Steps clear. Logic closed. No skipped steps. No omissions.

Time up. 07:40.

“Pens down. Hand in.”

The proctor collected the papers. Dense sound of paper rubbing paper.

Lin Chen set down his pencil. His fingers slightly stiff. Red marks pressed into the pads of them. He rolled his wrist. The joints gave a faint click.

He stood. Handed in the paper. Turned. Left.

The corridor was cold. Wind poured up from the stairwell. Carrying chalk dust and the smell of old wood.

Back to the dorm. Door pushed open. Locked. Canvas bag set on the bed. Movements light.

He spread open the ledger. Pencil moving.

Day 31. 08:10. Written exam finished. Estimated lost points: 1 multiple-choice question (fuzzy concept). 2 fill-in-the-blank questions (careless calculation). 1 calculation problem (redundant steps, possible process deduction). Estimated score: 82–85. Lesson: Need to strengthen transient processes in dynamic circuits. Must memorize the charging/discharging time constant in capacitive circuits: τ = RC. Funds: 0.00. Shortfall: soldering iron. Solder wire. Rosin.

The pencil tip paused. He closed the ledger.

He stood up. Went to logistics. Empty corridor. Echoing footsteps. Cold.

08:40. Logistics storeroom. Iron door half shut.

Administrator Old Zhao. Wrapped in an army coat. Sitting beside a coal stove. Ledger in hand. The fire glowed dim red. White vapor rose from the stovepipe.

“Lin boy. Borrowing tools?”

“Soldering iron. Solder wire. Rosin.”

Old Zhao raised an eyebrow. Set down the ledger. “We have stock. External-heating 30W. The tip is oxidized. Needs polishing. Solder wire. Leaded 63/37. Rosin. In blocks. You’ll need alcohol for cleaning.”

“Loan period. Deposit.”

“One day. Ten-yuan deposit. Overtime deducted by the hour. Damage paid at full cost.”

Lin Chen nodded. Reached into the pocket against his body. Empty.

“Not enough cash.”

Old Zhao sighed. “Rules are rules. No money, no loan. Or. Find a classmate and split the deposit.”

Lin Chen did not argue. Turned. Left. Stride length thirty centimeters. Did not step on the edge.

Back in the dorm. He spread open the canvas bag. Counted what he had. Dried tangerine peel. Old newspaper. Mercurochrome. Cotton swabs. Gauze. No usable substitute.

He closed his eyes. In his head there were no formulas. Only a circuit diagram. Temperature. Time. Molten solder. Solidifying.

He opened his eyes. His gaze fell under the bed. Old tin box. Inside were the parts he had traded for last time at the scrap depot. Resistors. Capacitors. Diodes. And a discarded length of heating wire.

He took out the heating wire. About fifteen centimeters long. Diameter 0.5 millimeters. Resistivity known. Power P = U²/R. If connected to 220V, it would need a series voltage drop. But the exam room had no 220V outlet. Only a 12V DC power supply.

He shook his head. Gave it up. Voltage mismatch. Insufficient power. The iron tip would not reach 350°C.

He pulled out the old notebook. Blank page. Pencil moving.

Alternative plans: 1. Borrow. Find someone in Group A. Share the deposit. Risk: social debt. Uncontrollable. 2. Modify. Old soldering iron + voltage regulator. No regulator in the exam room. Not feasible. 3. Cold solder. Conductive glue + crimping. Violates the rules. Zero score. 4. Low-melting-point alloy. Tin-bismuth alloy. Melting point 138°C. Can be heated with a 12V power supply. But requires a homemade iron tip.

The pencil tip paused. He closed the notebook. Fingers tightening. The page edges curled. He shoved it to the bottom of the bag and pinned it down.

He stood up. Went to the washroom. Cold water. Piercing. Scrubbed his face. The towel was rough. Rubbed his skin. Left red marks. Did not hurt. Awake.

Back in the room. He spread out a fresh sheet. Drew a sketch.

Soldering-iron tip. Copper rod. Diameter 6 millimeters. Length 50 millimeters. One end ground to a bevel. A hole drilled in the other end. Heating wire threaded through. Heating wire wound around the copper rod. Wrapped in insulating mica sheet. Connected to 12V power.

Calculated the power. Copper rod resistance extremely low. Needed a series current-limiting resistor. R = U²/P. Set P = 15W. R = 12²/15 = 9.6Ω. Choose a 10Ω/20W wirewound resistor.

Feasible. But needed materials. Copper rod. Mica sheet. Current-limiting resistor. Wire.

He reached into the pocket against his body. Empty. But the road was not cut off.

He took out the old ledger. Back side. Pencil moving.

Materials list: Copper rod. Scrap depot. 0.5 yuan. Mica sheet. Salvaged from an old radio. 0 yuan. Current-limiting resistor. 10Ω/20W. Electronics market. 1.2 yuan. Wire. Spare. 0 yuan. Total. 1.7 yuan. Funding shortfall. 1.7 yuan. Countermeasure: cafeteria at noon. Return half a dish. Exchange for cash. Or. Before the afternoon practical. Settle accounts with Old Sun.

The pencil tip paused. He closed the ledger.

He lay down. Closed his eyes. Breathing steady. Chest rising and falling. Fixed rhythm. In his head there were no formulas. No words. Only three lines. One was the wrong answers on the written exam. One was the sketch of the homemade soldering iron. One was the procedure for the afternoon practical. Three lines. Crossing in the dark. Not colliding. Not tangling. Each moving forward on its own.

He did not need to guess the difficulty. He only needed to prepare the tools. Copper rod. Resistor. Wire. Sandpaper. Electrical tape. Spare pencil leads. Three sticks. Draft paper. Two notebooks. Soldering notes. One page.

He closed his eyes. His fingers unconsciously rubbed the zipper pull of the canvas bag. The metal was cold. Rough. Like an unpolished stone.

The wind tightened. The mountain shadows in the distance darkened. Clouds pressed lower. Snow was coming again. He turned over. Faced the wall. His breathing slowed. Outside the window the snow grains grew finer, tapping against the glass with tiny pattering sounds. Everything was being smoothed flat. Only white remained. And cold.

Afternoon. Two o’clock. Practical.

He closed his eyes. Slept. But his hand was moving. Fingertips tracing circuit diagrams unconsciously across the bedsheet. Power supply. Current limiting. Load. Feedback. Node. Loop. Ground. Lines straight. Even. No shake.

Tick. Tick.

12:30. Cafeteria. Window. Glass partition. Heavy condensation.

“Half portion of cabbage. No soup.”

The server handed it over. Aluminum lunch tin. Steam rising faintly.

Lin Chen handed over twenty fen. The server gave change. Five fen. The coin was cold. A chip on the edge.

He took it. The five-fen coin. Set it in his palm. Weighed it. Light. But heavy.

Funds. 0.00 + 0.05 = 0.05 yuan.

Shortfall. 1.65 yuan.

He bit into the cabbage. Dry. Tough. Chewed it to pieces. Swallowed. Something in his stomach now. Not full. Just grounded. The warmth slid down his throat. Drove off the chill.

13:00. Back to the dorm. Organizing his things.

Canvas bag. Layered.

Upper layer. Textbook. Notebook. Pencils. Eraser. Ruler. Compass.

Middle layer. Homemade soldering-iron components. Copper rod. Resistor. Wire. Sandpaper. Electrical tape. Wrapped in old newspaper. Against damp.

Lower layer. Change of clothes. Mercurochrome. Cotton swabs. Gauze. Dried tangerine peel.

Zipper pulled shut. Canvas bag against his back. Weight even. Did not press the shoulder. Center of gravity centered.

He sat on the edge of the bed. Closed his eyes. Rehearsed.

13:50. Laboratory. Assemble.

14:00. Practical begins. Soldering. Time limit thirty minutes.

Procedure clear. No redundancy. Tolerance for error. Low. Time window. Tight.

He opened his eyes. Looked at the watch. 13:40.

He stood up. Went out.

Corridor. Footsteps. Sparse. Someone coughing. Someone turning pages.

He walked along the main path. Avoided the crowd. Old teaching building. Red brick. Wall skin peeling. Gray-white inner layer exposed.

He reached the lab building. Stopped. Iron door ajar. A light clink of metal came from inside. And low-voiced discussion. Rosin. Machine oil. Old rubber. Mixed smells. Cold. But familiar.

He pushed the door open. Cold air poured in. Carrying the smell of rosin and machine oil. A faint sting in the nostrils. But clear.

Long tables. Covered with equipment. Oscilloscopes. Multimeters. Resistance boxes. Wires. Disorderly. No labels. Deep scratches in the tabletops. Grain exposed.

Old Li stood behind the lectern. Stopwatch in hand. Metal case. Worn. Glass face. Pointer still. His gaze swept the room.

“Timing starts. Solder a voltage-regulator circuit. Requirements: bright solder joints. No cold joints. No burrs. Power-on test. Output voltage 5V ± 0.1V. Time limit thirty minutes. Overtime. Zero points. Short circuit. Compensation.”

The crowd fell silent at once. Only the sounds of rummaging for equipment. And quick breathing. Someone’s hand shook. Someone opened the wrong drawer. Someone cursed under his breath.

Lin Chen opened the canvas bag. Took out the components. Spread them out. Arranged in order. No panic. No scrambling. No touching anyone else’s things.

He picked up the copper rod. Polished it with sandpaper. Beveled face. Smooth. No oxide layer. Drilled a hole. Threaded the heating wire through. Wound it tight. Wrapped it in mica sheet. Fixed it with electrical tape. Connected it to the 12V power supply. 10Ω resistor in series.

Power on. The copper rod heated up. Faint red. He brought the block of rosin close. It melted. A little white smoke rose. The smell sharp. But familiar.

He picked up the solder wire. Leaded 63/37. Melting point 183°C. Touched it to the joint. Not the iron. Three seconds. Solder melted. Withdraw. Solidify.

Solder joint. Bright. Conical. No burrs. Pass.

Stopwatch. Tick. Tick.

Countdown. Twenty-nine minutes and fifty-nine seconds.

He closed his eyes. In his head there were no formulas. Only temperature. Time. Molten solder. Solidifying. Three lines. Crossing in the dark. Not colliding. Not tangling. Each moving forward on its own.

He opened his eyes. His fingers came down. Motion steady. No shake.

Old Li’s gaze passed over him. Paused. No comment. Only pressed the stopwatch. Tick. Tick.

Outside the window. The sky dark. Clouds low. No snow yet. Wind passing through the gaps. Giving a low howl.

The loudspeaker. Static. Zzzt. Old Li’s voice. Distorted. Penetrating the walls.

“Also. Notice from the Municipal Education Commission. The winter camp is adding an extra practical exam. Soldering. Bring your own soldering iron. Solder wire. Rosin. Anyone without them may borrow. Deposit deducted. Limit two loans per day. Overtime. Charged by the hour.”

The current cut off. Dormitory quiet.

Lin Chen opened his eyes. His gaze fell on the side pocket of the canvas bag. Dried tangerine peel. Old newspaper. He reached into the pocket against his body. Receipt. Red stamp. Fifty yuan. Already paid. But the road was not smooth. Soldering. He did not understand it. But he could learn. Graph paper. He had to draw it. Curves. He had to calculate them. Time. He had to squeeze it out.

He stood up. Went to the washroom. Cold water. Piercing. Scrubbed his face. The towel was rough. Rubbed his skin. Left red marks. Did not hurt. Awake. Back to the room. Spread out fresh graph paper. Pencil. Ruler. Drew axes. Marked graduations. Did not trace. Did not shade. The lines straight. Even. He closed his eyes. In his head there were no curves. Only solder joints. Temperature. Time. Molten solder. Solidifying. Three lines. Crossing in the dark. Not colliding. Not tangling. Each moving forward on its own.

He opened his eyes. His gaze fell on the paper. First stroke. Down. Steady. No shake.

Coordinate axes. Horizontal axis. Load current. Vertical axis. Output voltage. Origin. Zero point. Graduations. Each square 0.1A. 0.1V. He calculated. No-load 5.0V. Full load 4.95V. Voltage drop 0.05V. Linear. Negative slope. He connected the line. No bend. No break. Pencil dust fell on the paper. He erased it lightly. Left no trace.

His foot hurt. He changed posture. Shifted his weight to the right leg. Left leg suspended. Did not press the injury. His fingers had gone numb with cold. He breathed warm air on them. White mist dispersed. Kept drawing.

Outside the window. Wind stopped. The snow grains grew finer. They tapped against the glass with tiny pattering sounds. Everything was being smoothed flat. Only white remained. And cold.

Tomorrow morning. Six o’clock. Written exam. He needed sleep. But his hand was moving. Pencil tip rasping over the paper. Rustle. Like a stopwatch. Tick. Tick.

He took out the old notebook. Blank page. Pencil moving.

Soldering. Key points. 1. Soldering-iron temperature. 350°C. Too high. Oxidation. Too low. Cold joint. 2. Rosin. Flux. Prevents oxidation. Use little. Cover the leads. 3. Solder wire. Leaded. Low melting point. Touch the joint. Not the iron. 4. Time. Three seconds. Melt. Withdraw. Solidify. 5. Inspection. Bright. Conical. No burrs.

The pencil tip paused. He closed the notebook. Fingers tightening. The page edges curled. He shoved it to the bottom of the bag and pinned it down.

He lay down. Closed his eyes. Breathing steady. Chest rising and falling. Fixed rhythm. In his head there were no formulas. No words. Only three lines. One was the rule for estimating readings. At midpoint, estimate five. Slightly right, estimate seven. One was the frozen road. Rubber-sole tread. Shift center of gravity forward. Do not step on the edge. One was the soldering procedure. Temperature. Time. Molten solder. Solidifying. Three lines. Crossing in the dark. Not colliding. Not tangling. Each moving forward on its own.

He did not need to guess the difficulty. He only needed to prepare the tools. Graph paper. Pencil. Ruler. Eraser. Spare leads. Three sticks. Draft paper. Two notebooks. Soldering notes. One page.

He closed his eyes. His fingers unconsciously rubbed the zipper pull of the canvas bag. The metal was cold. Rough. Like an unpolished stone.

The wind tightened. The mountain shadows in the distance darkened. Clouds pressed lower. Snow was coming again. He turned over. Faced the wall. His breathing slowed. Outside the window the snow grains grew finer, tapping against the glass with tiny pattering sounds. Everything was being smoothed flat. Only white remained. And cold.

Tomorrow morning. Six o’clock. Written exam.

He closed his eyes. Slept. But his hand was moving. His fingertips traced coordinate axes unconsciously over the bedsheet. Horizontal axis. Vertical axis. Origin. Graduations. Lines straight. Even. No shake.

Tick. Tick.