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Let's say you have a 120-watt light bulb plugged into a wall socket. The voltage is 120 volts, and a 120-watt bulb has 1 amp flowing through it. You can calculate the resistance of the filament by rearranging the equation:

R = V/I,

So the resistance is 120 ohms.

Beyond these core electrical concepts, there is a practical distinction between the two varieties of current. Some current is direct, and some current is alternating -- and this is a very important distinction.

Direct Current versus Alternating Current

Batteries, fuel cells and solar cells all produce something called direct current (DC). The positive and negative terminals of a battery are always, respectively, positive and negative. Current always flows in the same direction between those two terminals.

The power that comes from a power plant, on the other hand, is called alternating current (AC). The direction of the current reverses, or alternates, 60 times per second (in the U.S.) or 50 times per second (in Europe, for example). The power that is available at a wall socket in the United States is 120-volt, 60-cycle AC power.

The big advantage that alternating current provides for the power grid is the fact that it is relatively easy to change the voltage of the power, using a device called a transformer. Power companies save a great deal of money this way, using very high voltages to transmit power over long distances.

How does this work? Well, let's say that you have a power plant that can produce 1 million watts of power. One way to transmit that power would be to send 1 million amps at 1 volt. Another way to transmit it would be to send 1 amp at 1 million volts. Sending 1 amp requires only a thin wire, and not much of the power is lost to heat during transmission. Sending 1 million amps would require a huge wire.

So power companies convert alternating current to very high voltages for transmission (such as 1 million volts), then drop it back down to lower voltages for distribution (such as 1,000 volts), and finally down to 120 volts inside the house for safety. As you might imagine, it's a lot harder to kill someone with 120 volts than with 1 million volts (and most electrical deaths are prevented altogether today using GFCI outlets). To learn more, read How Power Grids Work.

There's one major electrical concept left that we haven't discussed: grounding.

Electrical Ground

When the subject of electricity comes up, you will often hear about electrical grounding, or just ground. For example, an electrical generator will say, "Be sure to attach to an earth ground before using," or an appliance might warn, "Do not use without an appropriate ground."

It turns out that the power company uses the Earth as one of the wires in the power system. The planet is a good conductor, and it's huge, so it makes a handy return path for electrons. "Ground" in the power-distribution grid is literally the ground that's all around you when you are walking outside. It is the dirt, rocks, groundwater and so on.

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If you look at a utility pole, you'll probably be able to spot a bare wire coming down the side of the pole. This connects the aerial ground wire directly to ground. Every utility pole on the planet has a bare wire like this. If you ever watch the power company install a new pole, you will see that the end of that bare wire is stapled in a coil to the base of the pole. That coil is in direct contact with the earth once the pole is installed, and is buried 6 to 10 feet (2 to 3 meters) underground. If you examine a pole carefully, you will see that the ground wire running between poles are attached to this direct connection to ground.

Similarly, near the power meter in your house or apartment there is a 6-foot (2-meter) long copper rod driven into the ground. The ground plugs and all the neutral plugs of every outlet in your house connect to this rod. Our article How Power Grids Work also talks about this.

Explore the links on the next page to learn even more about electricity and its role in technology and the natural world.

How Faraday Cages Work

Electricity is the lifeblood of many aspects of our world. Without volts and amps, many of our technological innovations would cease to exist. Even our bodies wouldn't function without an electrical charge zipping through our cells. But what electricity gives, electricity can take away.

Although this form of energy is vital to so much of our lives, it's one of those things that are only good in the right amounts. Too much electricity can electrocute people. Likewise, it can kill our modern electronics and machines.

But thanks to Michael Faraday, the brilliant 19th-century scientist, and one of his namesake inventions, the Faraday cage, we humans have developed plenty of ways to control electricity and make it safer for our computers, cars and other inventions -- and for us, too.

Faraday cages shield their contents from static electric fields. An electric field is a force field surrounding a charged particle, such as an electron or proton.

These cages often look distinctly, well, cagelike. Some are as simple as chain-link fences or ice pails. Others use a fine metallic mesh. Regardless of their exact appearance, all Faraday cages take electrostatic charges, or even certain types of electromagnetic radiation, and distribute

them around the exterior of the cage.

Electromagnetic radiation is all around us. It's in visible and ultraviolet light, in the microwaves that cook our food and even in the FM and AM radio waves that pump music through our radios. But sometimes, this radiation is undesirable and downright disruptive. That's where Faraday cages come in.

As a Faraday cage distributes that charge or radiation around the cage's exterior, it cancels out electric charges or radiation within the cage's interior. In short, a Faraday cage is a hollow conductor, in which the charge remains

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on the external surface of the cage.

That basic function has plenty of fascinating uses in our electrically cluttered and technology-packed world. And although Faraday would eventually have his day, the backdrop for his invention actually has its roots in earlier times. So, where did the idea for these ultra-useful cages come from? Find out on the next page.

Franklin's First Findings

It was Ben Franklin who helped inspire many of the ideas behind Faraday cages. Franklin, of course, spent part of his illustrious career flying kites in thunderstorms in attempts to attract lightning and thus was already somewhat acquainted with the vagaries and concepts of electricity.

In 1755, Franklin began toying with electricity in new ways. He electrified a silver pint can and lowered an uncharged cork ball attached to a non-conductive silk thread into it. He lowered the ball until it touched the bottom of the can and observed that the ball wasn't attracted to the interior sides of the can. Yet when Franklin withdrew the cork ball and dangled it near the electrified can's exterior, the ball was immediately drawn to the can's surface.

Franklin was mystified by the interplay of electricity and the charged and uncharged objects. He admitted as much in a letter to a colleague: "You require the reason; I do not know it. Perhaps you may discover it, and then you will be so good as to communicate it to me."

Decades later, an English physicist and chemist named Michael Faraday made other pertinent observations -- namely, he realized that an electrical conductor (such as a metal cage), when charged, exhibited that charge only on its surface. It had no effect on the interior of the conductor.

Faraday reaffirmed this observation by lining a room with metal foil and then charging the foil with the use of an electrostatic generator. He placed an electroscope (a device that detects electrical charges) inside the room, and, as he anticipated, the scope indicated that there was no charge within the room. The charge just moved along the surface of the foil and didn't penetrate the room at all.

Faraday further examined this phenomenon with his famous ice pail experiment. In this test, he basically duplicated Franklin's idea by lowering a charged brass ball into a metal cup. As expected, his results were the same as Franklin's.

This concept has all sorts of amazing applications, but here's one that's relevant to anyone who's ever been in an airplane. Imagine flying in an airplane that's suddenly struck by lightning. This isn't a rare occurrence -

- it actually happens regularly, yet the plane and its passengers aren't affected. That's because the aluminum hull of the plane creates a Faraday cage. The charge from the lightning can pass harmlessly over the surface of the plane without damaging the equipment or people inside.

It's not shocking, really. It's just science. On the next page, you'll see how this clever kind of cage design really works.

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Electrostatic for the People

In order to understand how Faraday cages work, you need a basic understanding of how electricity operates in conductors. The process is simple: Metal objects, such as an aluminum mesh, are conductors, and have electrons (negatively charged particles) that move around in them. When no electrical charge is present, the conductor has roughly the same number of commingling positive

and negative particles.

If an external object with an electrical charge approaches the conductor, the positive and negative particles separate. Electrons with a charge opposite that of the external charge are drawn to that external object. Electrons with the same charge as the external object are repelled and move away from that object. This redistribution of charges is called electrostatic induction.

With the external charged object present, the positive and negative particles wind up on opposite sides of the conductor. The result is an opposing electric field that cancels out the field of the external object's charge inside the metal conductor. The net electric charge inside the aluminum mesh, then, is zero.

And here's the real kicker. Although there's no charge inside the conductor, the opposing electric field does have an important effect-- it shields the interior from exterior static electric charges and also from electromagnetic radiation, like radio waves and microwaves. Therein lies the true value

of Faraday cages.

The effectiveness of this shielding varies depending on the cage's construction. Variations in the conductivity of different metals, such as copper or aluminum, affect the cage's function. The size of the holes in the screen or

mesh also changes the cage's capabilities and can be adjusted depending on the frequency and wavelength of the electromagnetic radiation you want to exclude from the interior of the cage.

Faraday cages sometimes go by other names. They can be called Faraday shields, RF (radio frequency) cages, or EMF (electromotive force) cages.

No matter what you call them, Faraday cages are most often used in scientific labs, either in experiments or in product development. On the next page, you'll discover exactly how engineers put these ingenious shields to the test.

Faraday, the Modern Way

People use Faraday cages for a wide array of purposes -- sometimes in esoteric lab settings, other times in common products. Your car, for example, is basically a Faraday cage. It's the cage's effect, not the rubber tires, that protects you in case of a nearby lightning strike.

A lot of buildings act as Faraday cages, too, if only by accident. With their plaster or concrete walls strewn with metal rebar or wire mesh, they often wreak havoc with wireless Internet networks and cellphone signals.

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But the shielding effect most often benefits humankind. Microwave ovens reverse the effect, trapping waves within a cage and quickly cooking your food. Screened TV cables help to maintain a crisp, clear image by reducing interference.

Power utility linemen often wear specially made suits that exploit the Faraday cage concept. Within these suits, the linemen can work on high-voltage power lines with a muchreduced risk of electrocution.

Governments can protect vital telecommunications equipment from lightning strikes and other electromagnetic interference by building Faraday cages around them. Science labs at universities and corporations employ advanced Faraday cages to completely exclude all external

electric charges and electromagnetic radiation to create a totally neutral testing environment for all sorts of experiments and product development.

Intrigued? Keep reading, and you'll see other wild ways this simple cage effect is put to use for sophisticated purposes.

Cutting-edge Cages

Swing by a hospital and you'll find Faraday cages in the form of MRI (magnetic resonance scanning) rooms. MRI scans rely on powerful magnetic fields to create medically useful scans of the human body. MRI rooms must be shielded to prevent stray electromagnetic fields from affecting a patient's diagnostic images.

There are plenty of political and military uses for Faraday cages, too. Politicians may opt to discuss sensitive matters only in shielded rooms that can block out eavesdropping technologies. All modern armed forces depend on electronics for communications and weapons systems, but there's a catch --these systems are vulnerable to aggressive EMPs (electromagnetic pulses), which can be a result of a solar storm or even man-made EMP attacks. To safeguard critical systems, militaries sometimes use shielded bunkers and vehicles.

It's for this same reason that Faraday cages are a fond subject in the survivalist subculture. These people, who preach self-sufficiency and mistrust of governmental response in the face of human-caused or natural disasters, believe in shielding all important electronics using homemade Faraday cages. In the event that an apocalyptic cataclysm strikes, they'll still have their shortwave radios and other high-tech tools that could be lifesavers.

Even if you're not particularly concerned with doomsday scenarios, Faraday cages likely play a role in your life every day. These cages harness a basic principle of physics and help people all over the planet put those principles to use -- for safety, luxury, convenience and to help further evermore exciting technological advances.

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Battery, Electric

Battery, Electric, a device that produces electric current without the use of moving parts. Strictly speaking, a battery consists of two or more electric cells connected electrically to provide a single source of electricity. However, in common usage, the term battery is also applied to single, self-contained cells such as those used in flashlights and electronic watches.

Batteries are a source of direct current (DC). They are widely used to supply electricity for equipment ranging in size from hearing aids to automobiles, and to provide power for portable equipment and equipment at remote locations. Batteries are generally impractical, however, for large-scale uses, such as lighting streets and houses.

Cells that provide electricity by transforming chemical energy into electrical energy are called galvanic, or voltaic, cells. There are two major types: primary cells and secondary cells. A primary cell is one that requires replacement once the materials it contains are used by the chemical reactions that take place in the cell. With a secondary cell, the chemical reactions can be readily reversed to restore the materials used up in the cell. Secondary cells form the basis of storage batteries. A third type, called a fuel cell, uses outside materials that are continuously supplied to the cell.

Cells that convert the energy of visible light into electricity are called photovoltaic, or solar, cells. Thermoelectric cells produce electricity from heat energy; nuclear batteries produce electricity from the radiation emitted by radioactive substances.

The Primary Cell

All galvanic cells consist essentially of a negative electrode and a positive electrode in an electrolytic solution. The solution contains ions (electrically charged atoms or groups of atoms) that promote chemical changes in the electrode materials. When a cell is connected to an electrical circuit, electrons flow from the negative electrode through the circuit into the positive electrode. The flow of electrons, or electric current, is produced by a difference in potential between the electrodes. The difference in potential is measured in volts and is generally referred to as voltage. It is caused by an electromotive force (emf) resulting from the relative strength with which atoms of each electrode attract electrons. The atoms of the positive electrode exert a stronger attraction for electrons than do the atoms of the negative electrode. When a current is established between the two electrodes, chemical reactions at each electrode proceed spontaneously, supplying free electrons to the circuit from the negative electrode and, at the same time, taking up free electrons from the circuit at the positive electrode.

In chemical terms, the type of reaction that occurs at the negative electrode is called oxidation and the type of reaction that occurs at the positive electrode is called reduction. Together, they constitute an oxidationreduction, or redox, reaction. The oxidized material loses electrons and the material that is reduced gains them. These processes can best be illustrated by using the zinc-mercury cell as an example. In this cell, zinc atoms make up the negative electrode and mercuric oxide molecules the positive electrode. When the two electrodes are connected electrically, the zinc is oxidized and the mercuric oxide reduced.

A simplified description of the cell's operation begins with the oxidation of an atom of zinc, which loses its two outer electrons. The resulting positive zinc ion combines with a negative oxygen ion in the electrolyte. The two free electrons left in the negative electrode enter the external circuit and pass to the positive electrode. There they combine with a molecule of mercuric oxide, reducing it. The mercury breaks away from the oxygen as a neutral metal atom and the oxygen enters the electrolyte as a negative oxygen ion. The overall chemical reaction of the cell during discharge can be expressed as Zn+HgO—ZnO+Hg.

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The reactions in a cell continue until the materials in the cell are used up. In general, the larger a particular type of cell is, the more electrical energy it can deliver. However, such effects as the accumulation of waste products from the chemical reactions can decrease a cell's performance. Commercial cells are designed to lessen these effects, which are known collectively as polarization.

Most commercial primary cells use zinc as the negative electrode, a metallic oxide as the positive electrode, and an acidic or alkaline solution formed by water and an electrolyte as the electrolytic solution. Most primary cells today are dry cells—the electrolytic solution is in the form of a moist paste or is held in absorbent materials so that it is not free to flow.

Graphite or carbon black is often mixed with the metallic oxide to improve the conductivity of the positive electrode. The zinc is usually amalgamated—that is, alloyed with mercury. The amalgamation helps prevent any decomposition of water and the associated formation of hydrogen gas. The accumulation of hydrogen gas around the negative electrode is a chief cause of polarization. Between the electrodes there is a porous barrier of paper or other material, called the separator. The separator allows ions in the electrolyte to pass through it but keeps material of one electrode from reacting directly with the material of the other electrode.

Typical voltages for fresh commercial primary cells range from 1.0 to 1.5 volts. Batteries with higher voltages generally contain several cells connected in series—that is, one after the other in a circuit. A 9-volt battery, for example, will contain six cells of 1.5 volts connected in series. The voltage of a cell tends to decrease with amount of use and the strength of current drawn from the cell. It can also be affected by temperature. Over time, the materials in a cell slowly deteriorate, even if the cell is not used. The time during which an unused cell can still perform adequately is referred to as the cell's shelf life.

Primary cells are manufactured in a large variety of shapes and sizes. For consumer use, common shapes and sizes include cylindrical D (flashlight) and AA (penlight) cells and smaller button-shaped cells of various sizes. A discussion of typical examples of major kinds of primary cells follows.

The Carbon-Zinc, or Leclanche, Cell

was the original dry cell and is the most common type of primary battery used today. In the typical carbon-zinc cell, a small can made of zinc serves as both the cell's container and the negative electrode. The inside of the zinc can is lined with the separator and filled with a mixture of manganese dioxide, which serves as the positive electrode. The separator and manganese dioxide are saturated with the electrolytic solution—ammonium chloride, zinc chloride, and water. This solution is slightly acidic. A carbon rod is placed in the center of the cell within the manganese dioxide to improve the electrical conductivity of the cell. The rod makes electrical contact with a metal plate that is insulated from the zinc can and forms the positive terminal. The voltage of a carbonzinc cell tends to drop as the cell is being used and to recover between periods of use.

The Zinc Chloride, or Heavy-duty,

Cell is very similar to the ordinary carbon-zinc cell. The major difference is that its electrolyte consists chiefly of zinc chloride.

The Alkaline, or Alkaline Manganese,

Cell contains electrode materials similar to those used in the carbon-zinc cell. Unlike the carbon-zinc cell, it uses an alkaline electrolyte—potassium hydroxide. In a typical alkaline cell, the zinc is in the form of a fine powder and lies in the center of the cell surrounded by a layer of manganese dioxide. The separator lies between the materials. The entire cell is encased in a steel container.

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The Mercury, or Mercuric Oxide,

Cell provides a relatively large amount of energy per unit volume. For this reason small cells, such as button cells, are commonly mercury cells. In mercury button cells, the positive electrode, consisting of mercuric oxide, forms a layer at the bottom of the cell. The negative electrode, consisting of powdered zinc, forms a layer at the top. The electrode materials and the separator are infused with an alkaline electrolyte.

The Silver, or Silver Oxide,

Cell is ordinarily in the form of a button cell that is very similar to the mercury button cell. However, the positive electrode is composed of silver oxide instead of mercuric oxide.

The Lithium Cell

Lithium cells produce voltages higher than other primary cells, but they are relatively difficult and expensive to manufacture because lithium is very reactive chemically. The electrolyte usually consists of organic salts in a nonaqueous solution.

All lithium cells contain a negative electrode composed of lithium, but the composition of the positive electrode varies with different types. The most common lithium cells used for consumer products have a positive electrode composed of manganese dioxide.

The Daniell Cell,

once widely used for telegraphic equipment, is historically important but is little used today. In this cell, the positive electrode is made of copper, the negative electrode of zinc. A separate electrolyte is used for each electrode: copper sulfate for the copper electrode and zinc sulfate for the zinc electrode. The two electrolytes are separated by a porous barrier.

The Weston Cell

is not used as a source of power, but to provide a standard voltage for calibrating electrical instruments. A Weston Standard Cell uses mercury as the positive electrode and cadmium as the negative electrode with an electrolyte of cadmium sulfate. It produces 1.018636 volts at 68° F. (20° C.).

The Storage Battery

A storage battery is composed of one or more secondary, or storage, cells. This type of cell is rechargeable—that is, the chemical reactions that produce electricity in the cell can be readily reversed to restore the materials in the cell to their original condition. (The chemical reactions in a primary cell either are difficult to reverse or cause irreversible changes in the cell's internal structure.) A secondary cell is recharged by forcing an electric current through the cell in a direction opposite to that of the current produced by the cell itself. Devices called rechargers make it possible to recharge some secondary cells with household electrical current.

For some uses, secondary cells are recharged frequently to keep the battery at full charge. For others, the battery is used essentially like a primary battery and is allowed to run down" before it is recharged. Over time, a storage battery will fail because of deterioration of its parts or decomposition of its materials. Two major kinds of storage batteries are the lead-acid and nickel-cadmium batteries.

The Lead-Acid Battery

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is used in most automobiles, trucks, and other vehicles for the starter motor, ignition system, and accessory electrical equipment. Each secondary cell produces approximately 2 volts, and the total voltage of the battery (6 or 12 volts in the case of automobiles) equals the number of cells multiplied by two.

The most common lead-acid cell consists of two types of plates immersed in a solution of sulfuric acid and water. Perforated separators made of fiber glass, wood, or rubber help insulate the plates from each other. The negative plates are made of spongy (porous) lead. The positive plates are a lead grid filled with lead dioxide. As the battery discharges (gives off current), the surfaces of the lead dioxide and spongy lead plates gradually become lead sulfate and the sulfuric acid becomes increasingly diluted with water formed during the process. As these chemical reactions occur, the battery becomes progressively weaker.

An automobile battery is recharged by an electric current produced by an alternator or generator. A device called a voltage regulator controls the charging process and prevents overcharging. Overcharging causes the plates in the battery to deteriorate.

The state of charge of some lead-acid batteries can be checked by testing each cell with either a hydrometer or a voltmeter. The hydrometer measures the specific gravity of the sulfuric acid in a cell. The specific gravity indicates a cell's state of charge. A voltmeter indicates the state of charge of a cell by measuring the cell's voltage. The failure of one or more cells to reach an adequate state of charge may be an indication that the battery needs replacing.

Some batteries lose water in the electrolytic solution through evaporation or decomposition and have removable caps for their individual cells so that water can be added periodically. Calcium-lead batteries, commonly known as maintenance-free batteries, do not require additional water; the cells usually do not have caps but are permanently sealed.

The Nickel-Cadmium Battery

has a positive electrode composed of nickelic hydroxide, a negative electrode composed of cadmium, and an electrolyte of potassium hydroxide. Nickel-cadmium cells are available in many of the same sizes and shapes as primary cells. In one flashlight nickel-cadmium cell, the positive and negative electrodes are long plates wound in a spiral within the metal cylinder. Wound with the plates is a separator that lies between the plates.

The Fuel Cell

A fuel cell converts the chemical energy of a fuel directly into electrical energy. A typical cell contains two metal screens separated by a layer of material saturated with an alkaline or acid electrolyte. Hydrogen is fed to one side of the electrolyte layer, oxygen to the other side. As the gases react with the electrolyte, a voltage is produced between the screens. The hydrogen and oxygen used by the fuel cell can be supplied to the cell for continuous operation.

Photovoltaic Cells

Light generates an electric current when it falls on certain substances and releases electrons from their atoms. The electrons are then available to flow through a circuit.

A typical photovoltaic cell is the silicon solar cell. Each silicon cell is formed from a wafer of pure silicon to which selected impurities have been added. The surface that is exposed to light is treated with boron or a similar element. The rest of the silicon is treated with an element such as arsenic. When light strikes the boron-treated surface, it releases electrons which then tend to move into the arsenic-treated layer. The surface is the positive

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electrode; the arsenic-treated layer is the negative electrode. When the two electrodes are connected by a wire, current flows from the arsenic-treated layer through the wire to the boron-treated surface.

Solar batteries power most artificial satellites. On earth they are used to power communications equipment, navigational buoys, and pumping stations, particularly at remote locations. Because solar batteries operate only when the sun shines, they are usually used with storage batteries if continuous power is needed.

Thermoelectric Cells

A thermoelectric cell produces an electric current directly from heat through differences in temperature. A simple thermoelectric cell consists of two conductors made of different kinds of metal, joined together in two places (the junctions). One of the junctions is then heated (or cooled) with respect to the other. As the temperatures at the two junctions begin to differ, a current starts to flow through the circuit. The strength of the current depends on the kinds of metals used as conductors, and on how great a difference there is in the temperatures of the two junctions.

Nuclear Batteries

The source of energy in nuclear batteries is radiation from radioactive atoms. Several methods are used to convert nuclear energy into electrical energy in batteries.

Beta-emission Cells

Beta particles are electrons emitted from the nuclei of atoms. The emitter in a beta-emission battery is a source of pure beta particles, such as hydrogen 3, krypton 85, or strontium 90. As the electrons leave the emitter, they pass through a vacuum or a relatively unreactive material, such as plastic. Some of the electrons strike a collector made of carbon, and enter a circuit. Direct beta-emission batteries produce extremely high voltages (up to 500 volts or more) with extremely low current. They are used to charge capacitors.

Nuclear Photocell Batteries

use radioactivity to produce light, which in turn produces electric currents in photovoltaic cells. In one type, promethium 147, a radioactive byproduct of nuclear fission, is mixed with a phosphor and encased in transparent plastic. Electrons given off by the promethium cause the phosphor to glow, just as electrons in a television picture tube cause phosphors on the screen to glow. The light strikes a photovoltaic cell, which

transforms the light into electrical current.

History

The first battery was made in about 1800 by Count Alessandro Volta, an Italian scientist. It consisted of a stack of alternating zinc and silver discs with a brine-soaked separating material after every second disc. The voltaic pile, as the battery came to be known, has the disadvantage that its voltage quickly drops because of waste products that accumulate near the discs during discharge.

A cell that overcame this problem was developed by an English scientist, John F. Daniell, in 1836. An improved Daniell cell was a reliable source of electrical energy for telegraphy through the end of the 19th century.

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