The return to Mt. Aocang was like stepping from a vibrant, bustling marketplace into a silent, ancient temple. The air was thin and crisp, carrying the scent of pine and mist instead of salt and spices. The profound quiet of the mountaintop was a stark contrast to the constant hum of Liyue Harbor, and for a day, they simply rested, allowing the peace of the adeptal realm to settle back into their bones.
The following morning, Xianyun led Ren into her workshop. The air within hummed with latent power, and the half-finished inventions on the workbenches seemed to wait with patient anticipation. She unrolled the blueprint, its lines seeming stark and brilliant in the mystical light of the cavern.
"Explain it to me again, Ren," she said, her golden eyes fixed on the diagram. "Leave nothing out. I wish to understand the core principles, not just the assembly."
This time, Ren's explanation was different. The childlike wonder was gone, replaced by a quiet, focused authority that seemed far too old for his small frame. He was no longer just a child describing a clever idea; he was an engineer outlining a scientific principle.
"The heart of the system isn't the crystal," he began, his small finger tracing the power source. "The electro crystal is just a battery, a reservoir of raw energy. The real key is in the directed flow and the resistance. We need a path for the energy to travel—a conductor. This path must allow the energy to move easily, but not too easily."
He moved his finger to the tightly wound coil. "And this is the destination. We create a deliberate bottleneck here. A point of high resistance. The energy, when forced through this choke point, converts from its flowing state into a thermal state. It sheds its energy as heat. The amount of heat is directly proportional to the amount of energy you force through, and the amount of resistance in the coil. The regulator is just a gate that controls that flow. It's a very simple, very elegant system of energy conversion."
Xianyun listened, her expression unreadable. She was not hearing a child's explanation. She was hearing a lecture, a distillation of physical laws from another world, spoken with a clarity and confidence that was utterly captivating. She found herself looking not at a ten-year-old boy, but at a brilliant mind temporarily housed in a small, adorable vessel. It was a strange and exhilarating feeling.
"Very well," she said, a new level of respect in her voice. "The theory is sound. Let us begin with the first principle: the path. The conductor."
And so, their work began. The first two weeks were a masterclass in trial and error, a delicate dance between Ren's otherworldly knowledge and Xianyun's practical mastery of Teyvat's elements. Their first task was simply to tame the raw, untamed power of an electro crystal.
Xianyun levitated a fist-sized, violet crystal into a secure containment field of shimmering Anemo energy. It pulsed with a wild, chaotic light, miniature bolts of lightning occasionally arcing from its surface with a sharp crackle. This was not the gentle, refined energy that powered a Vision; this was raw, elemental force.
"Now, we need a wire," Ren stated, pointing at a spool of what looked like simple copper wire on a workbench.
"Let us test its properties," Xianyun replied. Using her adeptal arts, she drew out a length of the copper wire and attached one end to the containment field. The moment the connection was made, the wire didn't just conduct the energy—it vanished in a brilliant, violent flash of white light, leaving behind only a puff of acrid smoke and the lingering smell of ozone.
Ren flinched back, his eyes wide. "Okay. Too much power. Or the conductor is too weak."
"The energy flow is not stable," Xianyun observed, her brow furrowed in concentration. "It surges, like a mountain spring after a storm. We do not need a simple path; we need a riverbank, something to contain and direct the flow without being eroded by it."
This set the pattern for their days. Ren would propose a concept from his memory of physics. "We need something that conducts well but has a high melting point. Perhaps an alloy?" Xianyun would then translate this into the materials of Teyvat. She would use her knowledge to procure different metals—iron from the mines of Liyue, silver, even rare ores infused with faint traces of Geo energy.
She would draw them out into fine threads, and they would test each one. The iron wire glowed cherry red and melted. The silver conducted the energy almost too well, offering little resistance and heating up along its entire length. They spent days analyzing the results. Xianyun would use her powers to create small, sensitive instruments of Anemo energy, allowing her to "feel" the flow, to sense the surges and drops in power. Ren would watch, processing the data, comparing it to the scientific principles he knew.
"The energy needs to be insulated," he concluded after a particularly spectacular failure involving a small explosion. "The path needs a shell, something that doesn't conduct the energy at all, to keep the flow contained within the wire."
Xianyun's eyes lit up. "Silk Flowers," she mused. "Their fibers are remarkably resilient and hold no elemental affinity. When woven and treated with sap from a Glaze Lily, they become as strong as leather but remain perfectly pliable."
It took three more days, but they finally created their first working prototype of a cable. It was a core of a specially treated silver-iron alloy, wrapped in a thick, insulating sheath of woven, lacquered silk. When they connected it to the contained electro crystal, it worked. The cable remained cool to the touch, while a steady, powerful stream of elemental energy flowed invisibly within. Xianyun could feel it with her adeptal senses—a smooth, controlled river where before there had been a raging, unpredictable flood. They had done it. They had invented the insulated electrical wire.
With the cable finalized, they moved to the second, more critical phase: the heating coil. This was the heart of the machine.
"This part is different," Ren explained, pointing to the coiled section of the blueprint. "The cable is designed for low resistance, to move power efficiently. The coil needs to be the exact opposite. We need a material with high resistance, something that will fight the flow of energy and get hot, but not so hot that it melts itself."
Their experiments began anew. They took their newly invented cable and attached different materials to its end. A simple iron coil glowed brightly but quickly began to warp and sag from the intense heat. They tried alloys mixed with Cor Lapis, which proved incredibly durable but didn't heat up enough, its Geo properties seemingly grounding out much of the energy.
This part of the process was a delicate, intricate dance. Ren would calculate the ideal length and width based on his memory of formulas. "We need more surface area to dissipate the heat effectively. Let's try a flatter, ribbon-like wire, and wrap it more tightly."
Xianyun, with her divine craftsmanship, would then forge his theoretical designs into reality. Her slender fingers, capable of assembling the most complex adeptal machinery, would carefully shape and wind the stubborn metals. She would then connect the coil to their cable, and they would watch, measure, and analyze.
It was during one of these tests, as a new alloy coil began to glow with a steady, brilliant orange light—not melting, not warping, just radiating a powerful, consistent wave of warmth—that Xianyun paused. She looked at the glowing coil, the product of their combined efforts. Then she looked at Ren, who was staring at their success with a look of pure, unadulterated triumph, his glowing azure eyes reflecting the coil's fiery light.