STATIC ELECTRICITY IS a start, but the real power comes when you start to move electrons and electrical charges more systematically. Our electrical grid, the network we use to move energy around, is an astonishing resource. By pushing electrical charge down wires, and controlling it by using tiny switches and amplifiers, we can deposit energy wherever we need it. An electrical circuit is just a way of redistributing electrical energy. The most important thing about a circuit is that it is just that—a circuit. It has to be a loop, so that electrons can keep shuffling around it without building up at the far end. Every circuit must begin and end at a power supply, something that will keep the electrons on the move, taking them in at one end, pushing them along, and putting them back into the circuit at the other end. The power supply is a bit like an elevator that carries people up to the top of a very long slide. The people can go down the slide and up to the top again, around and around all day, as long as there’s an elevator to give them enough energy to get back to the start. The rule of every circuit is that you have to lose all the extra energy from the power supply before the electrons get back to where they started.
An electron shuffling along a wire is all well and good, but what’s pushing it around the circuit? We’ve said that the first thing is to have an electrical conductor, something that provides a path down which an electron can move. But the other thing you need is a force to push it with.
A fridge magnet and a balloon charged with static electricity are both weird for the same reason: They show that it’s possible to have an invisible force field. That is, one stationary object is pushing or pulling on another one nearby, but you can’t see what’s doing the pushing. This similarity isn’t accidental, but the real link is only obvious once you start moving the electrical or magnetic fields around. First, let’s go back to that principle of a force field. It’s not just humans that can make use of them.
The stream bed is a murky brown maze of rocks, plants, and tree roots. It’s dusk and the muddy water is flowing lazily through and over the obstacle course. A yard below the surface, two small antennae are poking out from beneath a pebble, twitching as they taste the water. Something moves nearby and the antennae vanish. This freshwater shrimp is a scavenger, hungry but vulnerable. Upstream, a hunter slides into the dark water. It paddles along the surface toward the center of the stream with two webbed front feet, then shuts its eyes, closes its nose, seals its ears, and dives. The platypus is ready for dinner.
If the shrimp stays perfectly still, it will be safe. The platypus swims quickly, picking its way confidently through the maze even though it’s currently blind, deaf, and unable to smell anything. Its flat bill sweeps from side to side, scanning the mud. Another foraging shrimp feels the water move as the platypus approaches and snaps its tail, jerking backward into the gravel. The hunter swerves toward it. The signal forcing the tail muscle of the shrimp to contract was an electric one. That electric pulse created a temporary electric field centered on the shrimp. This electric disturbance flashed through the surrounding water, exerting tiny pushes and pulls on nearby electrons. It lasted for a fraction of a second, but it was enough. A platypus has an array of forty thousand electrosensors on the upper and lower surfaces of its bill. The simultaneous water movement and electric pulse were all it needed to get a direction and range. The bill hammers into the sand in exactly the right place, and the shrimp is no more.
The shrimp’s movement condemned it because the act of moving changed its electric field. Every electric charge pulls or pushes on other electric charges around it. An electric field is just a way of describing how strong that push or pull is in different places, while talking about electric signals means that an electric charge moved somewhere, and something nearby noticed the change because the push on it increased or decreased. Since all muscle movements involve moving electric charges around inside the muscles, they all generate electric fields. So electrosensing is a reliable hunting technique underwater if you’re close enough to your prey, because no amount of colorful camouflage can disguise an electric signal. Any animal has to move eventually, and the tiniest motion will generate an electric signal that can give it away.
If that’s the case, why aren’t we more aware of the electric fields that we generate ourselves? It’s partly because those fields aren’t very strong, but mostly because electric fields decay quickly in air, which doesn’t conduct electricity. Stream water (and especially salty ocean water) is a very good conductor of electricity, so electric signals can be detected from much farther away. Almost all the species that use electrosensing are aquatic (bees, echidna, and cockroaches are the known exceptions).
In an electric circuit, the electrons move because there’s an electric field inside the wire. That electric field is pushing on each electron, shoving it along. But where does the electric field come from? A good place to start is with a battery. Batteries come in all shapes and sizes, but there is one set in particular that I will never forget. They were chunky sea batteries, and I worried about them because they were floating free in a giant storm, powering my one shot at an important experiment.
To study the physics of the ocean surface in storms, we need to go and look at that surface. The ocean is such a complicated environment that theorizing from a nice warm office is of limited use unless you’re sure that what you’re working on is definitely based on reality. But even when you get “there,” on a ship miles from shore in rough seas, it’s still difficult to touch the region I’m interested in—the water just a few yards below the sea surface. Knowing what happens there will improve our understanding of how the oceans breathe, and will contribute to better weather forecasts and climate models. But to see the details, you need to be in it; and it’s a violent, messy, dangerous place to be. I can’t swim in that water, but my experiments have to. The experiments need power, an electricity supply, and they need it while they’re bobbing up and down in the waves, free of the ship. You can’t plug them in, so you have to rely on batteries. And fortunately for me, electrical circuits work just as well when they’re bobbing up and down as they do when on dry land.