The Physics of War (17 page)

Back home Napoleon managed to assemble a new army of 350,000. But now his foes were unified; Russia, Prussia, Austria, Great Britain, and Spain had formed a new coalition. Nevertheless, Napoleon did have a few victories, but at the battle of Leipzig his army was reduced to seventy thousand. Paris was soon surrounded, and it was captured in March 1814. The victors exiled Napoleon to the island of Elba in the Mediterranean. Amazingly, he escaped from Elba in February 1815 and returned to the mainland, where he was hailed as a hero, and for another hundred days he again governed France. Then he met the British general Wellington at the Battle of Waterloo and suffered his greatest defeat. This time he was banished to St. Helens in the Atlantic Ocean. He died in 1821.

COUNT RUMFORD

Although few discoveries in physics were made in France during the Napoleonic era, important advances were being made elsewhere, and most of them were related to heat and thermodynamics. Count Rumford (his honorary title) was born Benjamin Thompson in Woburn, Massachusetts, in 1753. Early on he worked for the British army, conducting important experiments on gunpowder. His results were published by the Royal Society of England in 1781. He also began a series of experiments on heat about the same time.
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When the American Revolutionary War was over he left for London. After four years, however, he moved to Bavaria, where he continued his experiments on heat and light. He was recognized by the Bavarian government in 1791 and made a count of the Holy Roman Empire, with the title Count Rumford. The name he chose was the name of the town in New Hampshire where he had been married.

His major scientific interest for many years was heat. Early on he devised a method for measuring the specific heat of solid substances. Specific heat is the amount of heat needed to raise a given quantity (e.g., 1 gram) of a substance by one degree. He delayed publishing his results, however, and was disappointed when someone else published the same result before he did.

His most important discovery, however, took place while he was in Munich. He was put in charge of manufacturing brass cannons, and as he watched how they were being made he was amazed by the amount of heat that was produced by the boring. Water was used for cooling, but it boiled rapidly. He decided to measure the amount of heat produced in the process. Setting up a specially shaped cannon barrel that could be insulated against heat losses, he immersed the drill and barrel in a tank of water and measured the temperature increase of the water as the boring took place. This allowed him to determine how much heat was being produced. But he went a step further: he calculated how much heat was produced for a given amount of mechanical work. We now refer to this as a mechanical equivalent of heat. His value is somewhat higher than the value we now accept (4.18 Joules per calorie), but it was an important first step, and it established an important relationship in physics.

In performing the experiment Rumford also showed that no physical change had taken place in the material of the cannon, and the supply of frictional heat seemed to be inexhaustible as long as the boring continued.

Rumford also made important contributions to photometry, the measurement of light. In particular, he introduced the light unit, the standard candle.

NEW BREAKTHROUGHS IN PHYSICS

While wars were raging, other important discoveries were being made in physics, most notably in the study of electricity and magnetism. It would take years to understand the new phenomena thoroughly, and to apply them to useful devices, but there's no doubt that they eventually had a huge effect on warfare and weapons, and on everyday life.

In the early 1730s the French physicist Charles du Fay discovered that electrified objects are sometimes attracted to one another and sometimes repel
one another. He postulated that there are two types of electrical fluid, which he called vitreous electricity and resinous electricity (later called positive and negative electricity). He also noted that some materials conduct electricity better than others; he referred to this property as “contact electrification.”

A few years later, in 1746, the statesmen and scientist Benjamin Franklin became interested in electricity and carried out experiments using a Leyden jar (a jar with a brass rod down its center that could be used to store electrical charge). He wondered if the lightning bolts during thunderstorms were related to the sparks that could be produced near the ball at the top of the Leyden jar. To satisfy his curiosity he flew a kite during a thunderstorm, equipping it with a pointed wire connected to a silk thread, and tying a metal key to the other end of the thread. As expected, when he put his hand near the key, it sparked in the same way that Leyden jars did. Franklin was now sure that the “electric fluid” of the Leyden jar was also present in the storm clouds.

The French physicist Charles Coulomb began looking into the problem of the attraction and repulsion of electrified bodies in the early 1780s. He was particularly interested in the force between them. If they attracted or repulsed one another there had to be a force associated with the phenomena. He constructed a very sensitive device called a torsion balance that allowed him to measure the magnitude of the force, and he found that it was proportional to the inverse square of the distance between the two charges, and proportional to the product of the two charges. We now write this as F = q
₁
q
₂
/r
²
, where q
₁
and q
₂
are the magnitudes of the two charges and r is the distance between them.

The scene then switched to Italy, where the physician and physicist Luigi Galvani began to take an interest in the new field of “medical electricity” in about 1790. One day Galvani was skinning a frog on which he had been experimenting with static electricity. His assistant had touched a metal scalpel to a nerve on the frog's leg with a charged Leyden jar nearby. When the scalpel touched the nerve, the dead frog's leg jumped, as if alive. This observation surprised him, and he published it in 1791. He assumed that the jerking was caused by an electrical fluid in the nerves, and he called the phenomena “animal electricity.”
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Soon after the result was published, another physicist in Italy, Alessandro Volta, read about it and repeated the experiment. He noticed almost immediately that a frog was not needed; the only thing needed was two dissimilar metals and a moist conductor (they replaced the frog leg). And within a short time he went a step further, showing that a series of several bimetallic strips and moist conductors worked even better. Volta continued working on his new device, which he called a pile, using disks of silver and zinc on top of one another with cardboard disks soaked in salt water between them. With the new device, which we now know as
a battery, the continuous flow of electrical current was created for the first time.

Once scientists had a “current” of electricity flowing in a wire conductor, many physicists began experimenting with it. Among them was the German physicist Georg Ohm. Ohm soon found that the current that flowed along a wire between two points depended on the “resistance” of that section of wire. His law is now referred to as Ohm's law. Current is now measured in terms of a unit called an ampere, and resistance is measured in units called ohms. Mathematically, his law can be stated as V = IR, where V is the voltage between the two points, I is the current, and R is the resistance.
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The voltaic pile of Volta.

But there was still a serious problem. Because electricity had so many properties that were similar to magnetism, it appeared that they had to be related, but no one could prove it. In 1813 the Danish physicist Hans Christian Oersted became interested in the problem, but after experimenting for several years he was unable to find a connection between the two. Then one day in 1820 he was giving a lecture; during the lecture he was turning an electrical current off and on. Nearby
was a compass, and, to his surprise, he noticed that his actions were having an effect on the compass. He brought the compass up to the wire, holding the compass needle parallel to the wire. When he turned the current on, the needle moved to a perpendicular direction. As a result, he determined that an electrical current has a magnetic field associated with it. The magnetic field surrounded, or circled, the current-carrying wire, and the magnitude of the field weakened as the distance from the wire increased.
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Oersted published his results in July 1820, and they soon caused a sensation. There was now proof: electricity and magnetism were, indeed, related. In particular, an electric field would produce a magnetic field. It was also soon found that a moving magnet could produce electrical current. The interaction between the two fields is now referred to as electromagnetism.

Within a few weeks of the publication of Oersted's discovery, the French physicist Andre Ampere read about it. He verified Oersted's work and went on to conduct experiments on the fields around the wire. He showed that two parallel wires carrying electrical currents attracted or repelled each other, depending on whether the current flow was in the same direction or in opposite directions. Working out the details of the interaction, he showed that the forces between them obeyed an inverse square law. He then went on to develop the “right-hand rule” for current, which says that if you grasp a current-carrying wire with your thumb in the direction of the current and close your fingers, your fingers will point in the direction of the magnetic field. He was also the first to develop a solenoid—a coil of wire wound in a spiral that created a magnetic field down its center.

But perhaps the greatest shining light of the era was Michael Faraday, who was born in 1791. He was basically self-educated; at fourteen he was apprenticed to a local bookbinder, which brought him into contact with large numbers of books. He read as many of them as he could in his spare time, and he was particularly inspired by one that described the new phenomena of electricity. Later he attended lectures given by the eminent physicist Humphrey Davy.
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After reading about Volta's pile he constructed one for himself, and in 1821, after Oersted announced his discovery, Faraday built two devices that produce what he called “electromagnetic rotation.” They were simple versions of the electric motor. Then in 1830 he began asking himself if a magnetic field that was already in existence could produce an electrical current. To find out he wound a wire around an iron ring and attached the two ends to a battery, producing a solenoid, then he placed a switch in the circuit so that it could be turned on and off. On the opposite side of the ring he wound several loops of another wire and attached its ends to an instrument that measured current, called a galvanometer. Faraday then turned the switch off and on several times, expecting to see a current in the second wire. To his disappointment, however, there was only
a tiny current that lasted for a fraction of a second. Experimenting further, he finally determined that it was not the existence of the magnetic lines that created the current but rather the motion of the magnetic field across the wire. He soon showed that if he merely pushed a magnet into a coil of wire, it would produce a current in the wire. We now refer to this as electromagnetic induction.

The right-handed rule for the direction of the magnetic field.