Half a century ago, vacuum tubes were very common in audio amplifiers. A small voltage applied to the grid of a vacuum tube controls a relatively large current that drives the electromagnet in a speaker, creating movement and thereby sound. Modern solid-state amps are superior in cost, size and reliability, but many people still prefer the warm sound and mesmerizing orange-yellow glow of a tube amp.
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.