The glowing coil was their breakthrough, the bright orange heart of their invention beating with a steady, contained power. It was a testament to their shared efforts, a perfect fusion of otherworldly theory and Teyvat's material science. But it was just a component. Now, they had to build the body around the heart.
"The heat is constant," Ren observed, shielding his face from the wave of warmth with his small hand. "But we need to be able to control it. We need the regulator."
He picked up a charcoal stick and, on a clean patch of the workshop floor, sketched a simple diagram. "It's about changing the length of the resistive path," he explained, his voice losing its childish lilt and taking on the familiar, focused tone of an instructor. "Right now, the energy flows through the entire coil. But what if we could tap into the coil at different points?"
He drew a circular dial connected to a contact point that moved along the length of his sketched coil. "The dial is a switch. When it's at the lowest setting, it connects the circuit near the very end of the coil, making the energy travel through the entire resistive path for maximum heat. But if you turn the dial, the contact point moves, creating a shortcut. The energy will only travel through a small section of the coil before completing the circuit. A shorter path means less resistance, which means less heat. It becomes a variable resistor."
Xianyun listened, her mind effortlessly grasping the elegant mechanical logic. It was a principle of such beautiful simplicity that she wondered why no one in Teyvat had ever conceived of it. Mortals used dimmer switches for lamps by placing different densities of magically treated glass in front of the light source. Adepti simply willed the light to be dimmer. This mechanical solution, relying on pure physics, was a revelation.
Crafting the dial was a task of delicate precision, perfectly suited to Xianyun's skills. She forged a series of small, conductive contact points from their silver-iron alloy and embedded them at precise intervals along the length of the heating coil. The dial itself was a simple disc of polished wood, connected to a rotating arm that would make contact with the embedded points, opening and closing the circuits exactly as Ren had designed. It was far more intricate than a simple on/off switch; it was a conversation with the machine, allowing the user to request exactly the amount of warmth they desired.
"Now, we need to move the air," Ren said, looking at the assembled coil and regulator. "The coil heats the air directly around it, but that's inefficient. We need to pull cool air in, pass it over the coil, and push the warm air out. We need a fan."
This was a concept Xianyun was intimately familiar with. Anemo was her domain. She could create a whirlwind with a flick of her wrist. "An adeptal enchantment could create a permanent, silent breeze," she suggested.
"No, Master," Ren countered gently but firmly. "The entire point is that anyone can build this, and anyone can use it. It can't rely on adeptal power. The fan has to be mechanical, powered by the same crystal as the coil."
He sketched another diagram, this one showing a tiny portion of the electro crystal's energy being diverted to a small, primitive electric motor—another concept that made Xianyun's mind whirl with possibilities. The motor would spin a simple fan blade, creating a steady, gentle airflow. The design was a marvel of self-contained efficiency.
While Xianyun worked on the delicate internal mechanics of the motor, Ren focused on the housing. He drew up plans for a simple, rectangular box made of treated Sandbearer wood, which was known for its poor heat conductivity. He designed vents on the top and bottom for airflow and a sturdy base to keep it stable. But he added one more crucial feature, a detail born from the memory of a world reliant on replaceable parts and consumer convenience.
"This part is very important, Master," he said, pointing to a section on the side of his drawing. "The electro crystal can't last forever. Eventually, its energy will be spent. The user needs to be able to replace it easily."
He designed a small, hinged door on the side of the housing. Inside, he drew a perfectly shaped slot where the electro crystal would sit, with two simple, spring-loaded contacts to connect it to the internal circuitry.
"When the heater stops working," he explained, "the owner just opens the door, pulls out the old crystal, and slots a new one in. Just like that. The machine isn't disposable; only the power source is. It makes it sustainable."
Xianyun looked at the design for the detachable power source and felt a profound sense of admiration. This was not just the mind of an inventor; it was the mind of a pragmatist. He was thinking not only about how the machine would work, but how people would live with it. He was considering its entire lifecycle, from construction to maintenance. It was a level of foresight that was, frankly, astonishing.
The final assembly took an entire day. Each component, perfected over weeks of intense work, was carefully fitted into the wooden housing. Xianyun, with the precision of a master watchmaker, connected the wires, secured the fan motor, and mounted the heating coil. Ren acted as her supervisor, his small form perched on a stool, his glowing eyes scrutinizing every connection, ensuring it matched his mental schematics.
Finally, it was done.
It sat on the main workbench in the center of the workshop, unassuming and elegant in its simplicity. It was a simple box of dark wood, about the size of a travel chest, with a slatted vent on its top and a single, polished dial on its front. It was an object of quiet, practical beauty.
Xianyun picked up a fresh, fist-sized electro crystal, its violet light pulsing with untapped energy. She opened the small door on the side of the box, her movements full of a quiet reverence. She slid the crystal into its slot, where it clicked satisfyingly into place against the contacts. She closed the door.
A quiet, expectant silence filled the workshop.
She reached out a slender, steady hand and turned the dial. A soft click echoed in the cavern. For a moment, nothing happened. Then, a low, gentle hum started from within the box as the small fan began to spin. A faint, violet light could be seen glowing deep within the vents.
Then came the warmth.
It wasn't the raw, aggressive heat of a pyro slime or the scorching blast of a forge. It was a gentle, pervasive warmth that flowed from the top vent in a steady, invisible stream. It was the comforting heat of a hearth on a cold winter's night, the warmth of the morning sun after a long, chilly dawn.
Xianyun turned the dial further. The hum of the fan increased slightly, and the flow of warm air grew stronger, pushing back the inherent chill of the stone workshop. She turned it down, and the warmth subsided to a barely-there whisper of heat.
It worked. It worked perfectly.
Ren hopped down from his stool, his face split by a grin of pure, unadulterated triumph. He ran over and stood in front of the vent, letting the warm air wash over him, ruffling his messy hair. It felt like success. It felt like progress.
Xianyun stood back, her arms crossed, and observed her creation. No, their creation. It was a machine born of two worlds, a testament to what could be achieved when ancient wisdom embraced new ideas. A soft, genuine smile graced her lips. Guizhong would have been so proud.