{"product_id":"rowlanda-rowland-disc","title":"Rowlanda – Rowland’s Rotating Charged Disc Physics Sticker","description":"\u003cp\u003eRowlanda, a bearded experimenter, spins a glowing charged disc beside a compass.\u003c\/p\u003e\u003ch3\u003eThe Physics\u003c\/h3\u003e\u003cp\u003eIn 1876, in Helmholtz's Berlin laboratory, Henry Rowland charged an ebonite disc and spun it fast. A magnetic needle nearby deflected (Am. J. Sci. 15, 30, 1878). A moving charge makes a magnetic field, just as a current in a wire does: current is charge in motion.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eB ∝ q v\u003c\/strong\u003e\u003c\/p\u003e\u003cul\u003e\n\u003cli\u003eB: magnetic field near the disc (T)\u003c\/li\u003e\n\u003cli\u003eq: charge on the disc (C)\u003c\/li\u003e\n\u003cli\u003ev: rim speed (m\/s)\u003c\/li\u003e\n\u003c\/ul\u003e\u003cp\u003eIt answered whether ‘convection’ of charge counts as a current: it does.\u003c\/p\u003e\u003cp\u003e\u003cem\u003eHonest caveat:\u003c\/em\u003e The deflection was tiny, a fraction of Earth's field; the full Biot–Savart form is needed for real numbers.\u003c\/p\u003e\u003ch3\u003eOn the sticker\u003c\/h3\u003e\u003cp\u003eRowlanda spins a glowing cyan disc in one hand, with a brass compass beside it. Printed text: \u003cstrong\u003eROWLANDA\u003c\/strong\u003e. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.\u003c\/p\u003e","brand":"Printify","offers":[{"title":"2\" × 2\" \/ White","offer_id":50964287717479,"sku":"18420176781986595504","price":4.99,"currency_code":"USD","in_stock":true},{"title":"3\" × 3\" \/ White","offer_id":50964287750247,"sku":"24387178942451410421","price":6.99,"currency_code":"USD","in_stock":true},{"title":"4\" × 4\" \/ White","offer_id":50964287783015,"sku":"89831168674034573350","price":8.99,"currency_code":"USD","in_stock":true},{"title":"6\" × 6\" \/ White","offer_id":50964287815783,"sku":"29460759202125343878","price":11.99,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0596\/8746\/9159\/files\/13563050699017587184_1200.jpg?v=1793664579","url":"https:\/\/pixelatedphysics.com\/products\/rowlanda-rowland-disc","provider":"Pixelated Physics","version":"1.0","type":"link"}