The Rating Nut
A short story by Claude, an AI. The physics in it is real and checked by a script; see the end.
The church clock in Aldmoor lost five seconds a day every summer, and every summer someone wrote to the parish newsletter about it.
Ines had been looking after the clock for nineteen years. She climbed the tower on the first Monday of each month with a canvas bag: a torch, a rag, a tin of oil she almost never used, and a pocket radio that picked up the time signal if she held it against the louvres.
This June she brought Kit, who was fifteen and had been sent by his mother because the alternative was him sitting in his room. He climbed the last ladder like it had personally offended him.
"It's just a clock," he said at the top.
"It's a pendulum," said Ines, "with a clock attached."
The pendulum hung through a slot in the floor into the room below: a steel rod as thick as a pencil, and at the bottom a cast-iron bob the size of a loaf. It swung one way in a second and back the next, slow enough that you could say tick and tock out loud and not feel silly.
"Why's it slow, then," said Kit. Not quite a question.
Ines put her hand flat on the stone wall. It was warm. "Feel the rod."
He did, cautiously, as if it might bite. "It's warm too."
"Steel grows when it's warm. Not much. If this rod gets ten degrees hotter it grows by about a tenth of a millimetre." She held up a finger and thumb with nothing between them. "That."
"That's nothing."
"It's nothing. It makes the swing a little longer, and that makes every second a little longer. About five seconds a day."
Kit looked at the rod, then at her, with the expression of someone deciding whether he was being made fun of. "Five seconds a day off a tenth of a millimetre."
"Count it. Eighty-six thousand four hundred swings a day. Each one a hair late."
She knelt by the bob. Underneath it, on the threaded end of the rod, was a brass nut with notches cut round its edge. "Rating nut. Turn it this way, the bob goes up, the pendulum's shorter, the clock gains. One notch is about a second a day. I've marked where it lives in summer and where it lives in winter."
There were two scratches on the brass, a few notches apart. Kit leaned closer. Next to them, older and fainter, were other scratches in other places.
"Who made those?"
"Mr Danby, before me. And whoever was before him. They had different winters."
He was quiet for a while, which she took as a good sign. Then: "Why don't you just put a thing on it that does it automatically? There are clocks that fix themselves. Radio ones."
"There are," said Ines. "I've got one in my kitchen. It's never wrong and I've never once looked at it to see how the day was going."
She let him turn the nut. He did it too fast the first time and she made him go back, and the second time he did it properly, five notches, feeling each one click under his thumb.
"That fixes the rate," she said. "Not the minute and a half it's already lost since May." She showed him the setting dial, and he walked the hands forward while she held the radio against the louvres. At the pips he let go, and the radio and the bell agreed to within a breath.
"It'll drift again," Kit said.
"Next month. Then it'll get colder and I'll turn it back."
"That's so pointless."
"It's a bit pointless," Ines agreed. "Hand me the rag."
On the way down he stopped on the last ladder. "Is the scratch for summer at the same place every year?"
"Near enough. Hotter years it's one notch further."
"So you could tell which years were hot. From the nut."
She hadn't thought of it like that in a long time. "You could."
He said nothing else. But the next month, on the first Monday, he was waiting at the bottom of the tower before she was.
The physics, checked
A clock pendulum that swings one way each second (a period of 2 s) is about 99.4 cm long: L = g T² / 4π² with g = 9.81 m/s². A steel rod expands by about 12 millionths of its length per degree, so 10 degrees warmer makes it longer by 0.12 mm ("a tenth of a millimetre"). The period goes as the square root of the length, so the clock runs slow by half that fraction, 6 × 10⁻⁵, which over the 86,400 seconds in a day is 5.2 seconds. A real rating nut's step depends on its thread; "about a second a day per notch" is a plausible design, not a measured fact, and the bob's position change behind it is worked out in the check script.