Loudspeaker

Strong neodymium magnets pinch hard enough to break skin and wipe bank cards. Handle one at a time and keep them clear of anyone with an implanted device.

Rice claims lowering the actuating coil's impedance with low-resistance copper rings acting as a short-circuited secondary in the pole pieces. Note that the cone is not what is claimed.

Roll a paper cone and hold it to a small sounder. Play music through it and note what is missing. This is the baseline the invention had to beat.

Wind about 40 turns of thin enamelled wire around a 25 mm cylindrical former. Keep the winding tight and even — a loose coil rubs in the gap and buzzes.

Record the DC resistance. It should be a few ohms. Zero means a short between turns; open means a break at a solder joint.

The coating is an insulator. Unscraped ends read open circuit and the commonest build failure is right here.

Cut a 150 mm paper disc, remove a wedge and join the edges into a shallow cone. Stiff and light — stiffness stops it flexing into its own resonances.

Fix the former to the cone's centre, square and centred. Any tilt will drag on the magnet later.

Fit a corrugated paper ring from the cone rim to a frame. It must be free along the axis and stiff sideways — that constraint keeps the coil out of the pole faces.

Position a disc magnet so the coil hangs around it without touching anywhere through its travel. Spin the cone gently and listen for rubbing.

Drive the coil with a 40 Hz signal at low level. The cone visibly pumps in and out. Current in a field becomes motion — nothing else is happening.

Sweep 50 Hz to 10 kHz and note where the output sags or peaks. Compare the evenness against the horn in step 3.

Record the coil's impedance at several frequencies. It rises with frequency — the coil's own inductance fights the drive, and the treble weakens.

Fit a closed copper ring around the pole near the coil and re-measure step 13. The impedance rise flattens: the shorted ring opposes the changing flux. This is the actual patent.

The patent. US 1,707,570, "Loud-speaker", filed 20 April 1925 and granted 2 April 1929, assigned to General Electric.

🔴 It names Chester W. Rice alone. This device is universally called the Rice-Kellogg loudspeaker, and the collaboration between Chester W. Rice and Edward W. Kellogg at GE is real and well documented — they presented the work together in 1925, and it reached the public as RCA's Radiola Model 104. But Edward W. Kellogg is not an inventor on this document. He filed separately (Serial No. 702,455) and the two also had a joint application (Serial No. 1,438), both referenced from within this patent. So "the Rice-Kellogg patent, US 1,707,570" is wrong in a small and very common way: the partnership is genuine, the co-inventorship on this particular number is not. Cite the pair for the work and Rice for this patent.

Why the copper ring is the claim and the cone is not. Moving-coil transducers were not new in 1925 — the principle dates to Ernst Siemens in the 1870s, and Oliver Lodge patented a moving-coil arrangement in 1898. What was missing was a design that stayed flat across the whole audio band. A voice coil is an inductor, and an inductor's impedance climbs with frequency, so a constant voltage delivers less and less current as the music goes up — the treble fades for purely electrical reasons before the cone even gets a chance. Rice's shorted copper rings sit in the magnetic circuit and act as a single-turn secondary: as flux tries to change, current in the ring opposes the change, flattening the inductance the coil presents. Step 13 and step 14 are the before-and-after of exactly that. Modern drivers still do this, and still call the part a shorting ring or Faraday ring.

What it unlocked. A speaker that reproduces the full range without a horn is the precondition for radio and recorded music becoming domestic objects rather than novelties: a horn is large, directional and coloured, and a cone driver is none of those. Public address, cinema sound, and eventually every recording that expected to be heard as the musicians intended, all sit downstream. The design has proved extraordinarily durable — a coil, a gap, a cone and a compliant surround is still what is in almost every speaker made a century later, and the build above is that same object at bench scale.

Its honest weakness. Direct radiation is inefficient. A horn couples the diaphragm to the air far better and can be many times louder for the same power, which is why horns never disappeared from stadiums and cinemas. Rice's design trades efficiency for accuracy — and, once amplification was cheap, that was the right trade.