TITLE Iron Filings PROMPT A desk toy: a sheet of white paper dusted with iron filings, with magnets you slide around underneath. Full-bleed Canvas 2D (a static paper layer plus a live layer for magnets, filings and instruments). Concept: make an invisible field visible; the delight is how thousands of tiny slivers swing into alignment as a magnet passes. Mood: clinical-playful, a physics classroom demonstration photographed from above in soft daylight. Palette: paper #ECEBE6 with faint fibre texture, paper tooth and a soft vignette, filings near-black #1A1A1A (with slight grey variation), magnets painted classic red #D62828 (north) and blue #1D4E89 (south) with a white "N"/"S" stamped on them, soft grey drop shadows, brushed-steel greys for the compass and shaker, pencil blue #3A68AA for diagram lines. Light page. Printed worksheet details: corner registration marks and a small "0 - 5 CM" scale bar. Simulation: up to 14,000 filings on desktop (scaled to area; about 4,800 on phones), each a short line segment roughly 4-9px with a position and an angle, held in typed arrays. The field at any point is the sum of softened monopole fields from each magnet's poles (two per bar and horseshoe; the disc is a single upward-facing pole giving a radial pattern). Every frame each filing eases its angle toward the local field direction with its own lag, scaled by field strength, so alignment spreads outward as a visible wave; about a third of the filings also creep along the gradient of field strength toward the poles when the pull is strong (clumping into spiky tufts at the poles and leaving emptier halos, and being dragged along by a moving magnet), with friction so they settle. Filing darkness, weight and length are modulated by field strength (dithered so there are no visible bands) so lines of force stand out as denser streaks. A "tap the paper" action (tap any empty paper, the red TAP PAPER button, or T) sends a ring outward from the tap point that jitters every filing it passes and briefly lowers friction, so they re-settle into cleaner lines, exactly as you would flick the sheet. Objects: the entrance demonstrates the signature moment: on load two bar magnets glide in from opposite sides (left/right on wide screens, top/bottom on tall ones) with opposite poles facing, and the filings, initially random, snap into the classic pattern with a taut bridge of lines across the gap within the first two seconds; a compass then pops in. A moulded tray along the bottom edge (side on wide screens) holds the rest: the slot for the second bar magnet (press to return it or take it out again), a horseshoe magnet, a small disc magnet, and a "fresh shake" shaker (press to re-dust the whole sheet in a top-to-bottom sweep, or drag it over the paper to sprinkle filings where you want them). Magnets can be clicked or dragged out of the tray and dropped back on it. Drag magnets (pointer/touch) with a slight lag and a soft shadow lift; rotate with a small handle that appears beside the magnet, the mouse wheel, a two-finger twist, or Q/E; double-click/tap flips polarity with a little flip animation. Bringing like poles together shows the null point between them, which gets a hand-drawn pencil circle and the note "null point"; opposite poles pull a taut bridge of lines. Extras that reward attention: a small steel compass drawn on the canvas, with a red-white needle that swings with damped oscillation, can be dragged around and always points along the field; toggling "field lines" overlays thin pencil-blue streamlines traced from the poles, like a teacher's diagram, with tiny arrowheads, drawn in progressively; a label strip bottom-left reads out the filing count, pole count, and the field strength and bearing at the compass; an optional sound switch (off by default) adds a faint gritty hiss proportional to how much the filings are moving, a thump for taps and a click for flips. Typography: sparse, like a lab worksheet: top-left a slip of paper taped to the sheet carries a small label in a typewriter/monospace face (American Typewriter/Courier stack) reading "Demonstration 14 — Lines of Force" (typed on at load), with a handwritten-style pencil note beneath in italic ("drag the magnets. tap the paper to settle the filings." plus one line about turning and flipping suited to mouse or touch); tool labels in 10px tracked caps. Performance: typed arrays, batch line drawing in six paths bucketed by shade, DPR capped at 2, the loop sleeps when everything has settled and pauses when the tab is hidden. Reduced motion: filings re-orient instantly without creeping or jitter animation, and magnets start in place. Keyboard: Tab to a magnet or the compass, arrows to move, R to rotate, F to flip, Delete to return it to the tray; tray tools and switches are ordinary buttons. DESCRIPTION A top-down view of a classroom demonstration: a sheet of warm white paper, fibres catching soft daylight, dusted with fourteen thousand black iron slivers. Two red-and-blue bar magnets glide in from opposite edges and the random dust visibly combs itself into the textbook pattern, pulling a dense bridge of filings across the gap between north and south. From there it is a toy: slide, spin and flip the magnets and watch the alignment wave chase them, see tufts of filings cling to the poles and get dragged along leaving swept trails, flick the paper to tidy the lines, bring like poles together to find the pencil-circled null point, and drop a horseshoe or disc magnet, a compass, or a fresh shake of filings from the apparatus tray. A taped typewritten label and a pencilled note are the only words on the sheet. VISUAL TECHNIQUES - Two stacked Canvas 2D layers: a procedurally painted paper (fibre strokes, noise tooth, daylight wash, vignette, printed registration marks and scale bar) and a live layer - Filing simulation in typed arrays: summed softened-monopole field, per-filing lagged axis alignment, gradient-of-field-strength creeping with friction and per-filing stopping thresholds - Batched line rendering in six paths bucketed by dithered field strength and grey variation, with length and weight tied to field strength - Magnets, compass and shaker drawn in code with painted-metal gradients, stamped lettering, bevel highlights and lift-dependent drop shadows - Flip animation by axis scaling with polarity swap at the midpoint and the field strength following the turn - Damped-spring compass needle driven by the local field - Midpoint-integrated streamlines seeded around each pole with arrowheads and a progressive draw-in, in pencil blue - Automatic null-point detection between like poles with a hand-drawn pencil annotation - Outward-travelling tap ring that jitters filings as it passes - Time-based tweens with custom easing for the entrance, tray hand-offs and tray/label choreography in CSS - Web Audio noise hiss mapped to filing motion, off by default - Sleep-when-settled render loop, DPR cap, resize rescaling, reduced-motion variant INTERACTION MODEL Drag any magnet or the compass with mouse or finger; it follows with a slight lag and lifts on a deeper shadow while the filings swing to follow. Rotate a bar or horseshoe magnet with the small handle beside it, the mouse wheel while hovering, a second finger twisting, or Q/E; double-click or double-tap flips its poles. Tap empty paper (or press TAP PAPER, or T) to flick the sheet and settle the filings into cleaner lines. Press a tray slot to place that magnet on the paper or call it back, or drag it straight out of the tray and drop it back on the tray to put it away; press the shaker for a fresh dusting or drag it across the paper to sprinkle. The FIELD LINES switch (or L) overlays the pencil diagram, and SOUND turns on the quiet filing hiss. With the keyboard, Tab reaches each object on the paper: arrow keys move it (Shift for larger steps), R rotates, F flips, Delete returns it to the tray; a dashed pencil-blue outline shows what is selected.