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EXTERNAL EARSThey guide sound to the sensitive middle ear

Looking more like a baby salamander than anything else, a six-week-old human embryo has tiny paddles for hands, dark dots for eyes and on either side of its shallow mouth slit, half a dozen small

bumps destined to form an ear. By nine weeks, these “hillocks” will migrate up the face as the jaw becomes more pronounced and start taking on the recognizable shell shape so handy for holding up eyeglass-es. Because development often reprises stages of evolution, the growth of embryonic ears in tandem with the jaw is no accident: the sound-trans-mitting middle ear bones that are a distinguishing feature of mammals evolved from what used to be gill arches in fi sh and jawbones in reptiles.

The tympanic membrane, or eardrum, that sits just outside the middle ear evolved separately and repeatedly in the ancestors of frogs, turtles, lizards, birds and mammals. Reptilian eardrums can do no more than crudely transmit low-frequency vibrations. To mammals, which have a fancier middle-ear setup, higher-frequency sounds are also audible; external skin and cartilage fl aps, called pinnae, are thought to have evolved to capture and funnel those sounds more effectively. The entire human ear structure amplifi es sounds by only about 10 to 15 decibels, but our pinnae also usefully modulate the frequency of sounds entering the ear canal. As the contours of the pinnae refl ect incoming vibrations, they slightly delay the higher-frequency sounds in a way that cancels out some of them. This so-called notch-fi ltering effect preferentially delivers sounds in the range of human speech to the inner ear.

Pinnae also help to detect where a sound comes from. Perhaps no animal has a keener directional hearing sense than bats, whose pinnae range in shapes and sizes tailored to the frequencies of each species’ own sonar signals. Another night hunter that relies heavily on hearing, the barn owl, instead uses its large ruff of facial feathers to capture sound and clues to its source. Studies of how human pinnae fi lter and refl ect sounds are informing the design of hearing aids to better reproduce natural aural mechanics. Robots and automated surveillance cameras that turn toward the sound of a disturbance are also being modeled on the human head and external ears. —Christine Soares

BATTERIESTheir inventor may not have known how they actually work

A battery’s power comes from the tendency of electric charge to migrate between different substances. It is the power that Italian scientist Alessandro Volta sought to tap into when he

built the fi rst battery at the end of 1799.Although different designs exist, the basic structure has remained

the same ever since. Every battery has two electrodes. One, the anode, wants to give electrons (which carry a negative electric charge) to the other, the cathode. Connect the two through a circuit, and electrons will fl ow and carry out work—say, lighting a bulb or brushing your teeth.

Simply shifting electrons from one material to another, however, would not take you very far: like charges repel, and only so many electrons can accumulate on the cathode before they start to keep more electrons from joining. To keep the juice going, a battery balances the charges within its innards by moving positively charged ions from the anode to the cathode through an electrolyte, which can be solid, liquid or gelatinous. It is the electrolyte that makes the battery work, because it allows ions to fl ow but not electrons, whereas the external circuit allows electrons to fl ow but not ions.

For example, a charged lithium-ion battery—the type that powers cell phones and laptop computers—has a graphite anode stuffed with lithium atoms and a cathode made of some lithium-based substance. During operation, the anode's lithium atoms release electrons into the external circuit, where they reach the more electron-thirsty cathode. The lithium atoms stripped of their electrons thus become positively charged ions and are attracted toward the electrons accumulating in the cathode. They can do so by fl owing through the electrolyte. The ions’ motion restores the imbalance of charges and allows the fl ow of electricity to continue—at least until the anode runs out of lithium.

Recharging the battery reverses the process: a voltage applied between the two electrodes makes the electrons (and the lithium ions) move to the graphite side. This is an uphill struggle, energetically speaking, which is why it amounts to storing energy in the battery.

When he built his fi rst battery, Volta was trying to replicate the organs that produce electricity in torpedoes, the fi sh also known as electric rays, says Giuliano Pancaldi, a science historian at the Universi-ty of Bologna in Italy.

Volta probably went by trial and error before settling on using metal electrodes and wet cardboard as an electrolyte. At the time, no one knew about the existence of atoms, ions and electrons. But whatever the nature of the charge carriers, Volta probably was not aware that in his battery, the positive charges moved in opposition to the “electric fl uid” moving outside. “It took a century before experts reached a consensus on how the battery works,” Pancaldi says. —Davide Castelvecchi

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