Text A
Maglev　Ⅱ

A magnetic levitation train，or maglev，is a type of train that is suspended in the air above a single track，and propelled using the repulsive and attractive forces of magnetism.Because of the lack of physical contact between the track and the vehicle，the only friction is that between the carriages and the air.As a result maglev trains can potentially travel at very high speeds with reasonable energy consumption and noise levels.Although the very high maximum speeds make maglev trains potential competitors to airliners on many routes，the high cost of the lines has limited their current commercial application to one line in Shanghai that takes people 30km to the airport in just 7minutes 20 seconds（top speed of 431km/h，average speed 250km/h）.Other maglev applications worldwide are being investigated for feasibility.

Technology

There are three main types of maglev technology：one that relies on superconducting magnets（electrodynamic suspension or EDS），one that relies on feedback controlled electromagnets（electromagnetic suspension or EMS），and a newer potentially more economical system that uses permanent magnets（Inductrack）.

Japan and Germany are active in maglev research，producing several different approaches and designs.In one design，the train can be levitated by the repulsive force of similar poles or the attractive force of opposite poles of magnets.The train can be propelled by a linear motor on the track or on the train，or both.A lot of electrical induction coils are placed along the track in order to produce the magnetic field necessary to propel the train，leading some to speculate that the cost of constructing such tracks would be enormous.

Unmoving magnetic bearings using purely electromagnets or permanent magnets are unstable on account of Earnshaw’s theorem；on the other hand diamagnetic and superconducting magnets can support a maglev stably.Conventional maglev systems are stabilized with electromagnets that have electronic stabilization.The electromagnets and electronics tend to be large，power-hungry and expensive.

The weight of the large electromagnet is a major design issue.A very strong magnetic field is required to levitate a large train，so conventional maglev research is using superconductor research for an efficient electromagnet.

The effect of a powerful magnetic field on the human body is largely unknown.For the safety of the passengers，shielding might be needed，which would add supplementary weight to the train.The concept is simple，but the engineering and design aspects are complex.

A newer less-expensive system is called“Inductrack”.The technique has a load-carrying ability related to the speed of the vehicle，because it depends on currents induced in a passive electromagnetic array by permanent magnets.In the prototype，the permanent magnets are in a cart；horizontally to provide lift，and vertically to provide stability.The array of wire loops is in the track.The magnets and cart are unpowered，except by the speed of the cart.At first inductrack was developed as a magnetic motor and bearing for a flywheel to store power.With only slight design changes，the bearings were unrolled into a linear track.Inductrack was developed by physicist Richard Post at Lawrence Livermore National Laboratory.

Inductrack uses Halbach arrays for stabilization.Halbach arrays are arrangements of permanent magnets that stabilize moving loops of wire without electronic stabilization.Halbach arrays were originally developed for beam guidance of particle accelerators.They also have a magnetic field on the track side only，therefore reducing any potential effects on the passengers.

At present，some space agencies，such as NASA，are researching the use of maglev systems to launch spacecraft.In order to do so，the space agency would have to get a maglev-launched spacecraft up to escape velocity，a task that would otherwise require elaborate timing of magnetic pulses or a very fast，very powerful electric current.Maglev-launching could also be used to make conventional launches more efficient：accelerating a craft up to Mach 1 before firing the main engines can save 30％ of the weight of the launch vehicle.

Pros and Cons of Different Technologies

Each implementation of the magnetic levitation principle for train-type travel involves advantages and disadvantages.Time will tell which principle and whose implementation wins out commercially.

New Words and Expressions

magnetic［mæɡ′netik］　adj.　of magnetism　磁性的，磁的

levitation［'levi′teiʃən］　n.　rising and floating in the air，esp.by means of supernatural powers　升空飘荡（尤指超自然力）

repulsive［ri′pʌlsiv］　adj.　（physics）causing repulsion；repelling　排斥的；斥力的

magnetism［′mæɡnitizəm］　n.　（science of the）properties and effects of magnetic substances　磁性；磁性作用；磁学

potential［pəu′tenʃəl］　adj.　①that can or may come into existence；possible　可能存在或出现的；可能的

②in existence and capable of being developed or used　潜在的；有潜力的

reasonable［′ri：zənəbl］　adj.　moderate；acceptable；tolerable　适度的；可接受的；还可以的

practical［′præktikəl］　adj.　suitable for the purpose for which it was made；useful　切合实际的；实用的

superconductivity［′sju：pə'kɔndʌk′tiviti］　n.　property of certain metals，at temperatures near absolute zero，of having no electrical resistance，so that once a current is started it flows without a voltage to keep it going　超导电性

electromagnet［i'lektrəu′mæɡnit］　n.　piece of soft metal that becomes magnetic when an electric current is passed through the coil surrounding it　电磁铁；电磁体

economical［'i：kə′nɔmikəl］　adj.　careful in the spending of money，time，etc.and in the use of resources；not wasteful　经济的；节俭的；省时间的；节约的

permanent［′pə：mənənt］　adj.　lasting or expected to last for a longtime or forever　永久的；永恒的；长久的；长期的

pole［pəul］　n.　（physics）either of the two ends of a magnet or the terminal points of an electric battery　磁极电极

linear［′liniə］　adj.　of or in lines　线的；线状的

speculate［′spekjuleit］　v.　form opinions without having definite or complete knowledge or evidence；guess　思考；思索；推断；推测

bearing［′beəriŋ］　n.　a device that supports，guides，and reduces the friction of motion between fixed and moving machine parts　轴承

diamagnetic［'daiəmæɡ′netik］　adj.　relating to or exhibiting diamagnetism；slightly repelled by a magnet　抗磁的

stabilization［'steibilai′zeiʃən，-li-］　n.　the act of stabilizing something or making it more stable稳定

shield［ʃi：ld］　v.　protect sb./sth.from harm；defend sb./sth.from attack，etc.　保护或庇护某人（某事物）

induce［in′dju：s，in′du：s］　v.　bring about，cause　引起，产生

array［ə′rei］　n.　an orderly arrangement　一系列

unroll［'ʌn′rəul］　v.　（cause sth.to）open out from a rolled position by rolling　（使某物）（由卷曲状态）展开

Mach［mɑ：k］　n.　（followed by a number）ratio of the speed of sth.（esp.an aircraft）to the speed of sound　马赫

spacecraft［′speiskrɑ：ft，-kræft］=spaceship　n.　vehicle manned or unmanned for travelling in space　航天器；宇宙飞船；太空船

pulse［pʌls］　n.①single vibration of sound，light，electric current，etc.　（声波、光波、电流等的）一次脉动

②（usu.sing）series of these　连续的脉动；脉冲波

implementation［'implimen′teiʃən］　n.　putting into effect；carrying out　使（某事物）生效，履行；实施；贯彻

inherent［in′hiərənt］　adj.　existing as a natural or permanent feature or quality of sb./sth.　内在的；固有的；本来的

fail-safe［′feilseif］　adj.　（of equipment，machinery，etc.）designed to compensate automatically for a breakdown or failure　（指设备、机器等）设有自动保险装置的

wheel［hwi：l］　v.　move in a curve or circle　转动，旋转

helium［′hi：liəm］　n.　chemical element，a light colourless gas that does not burn，used in airships　氦

repulsive force　the force by which bodies repel one another　斥力；排斥力

linear motor　a form of electric motor in which the stator and the rotor are linear and parallel.It can be used to drive a train，one part of the motor being in the locomotive，the other in the track　线性电动机（定子和轮子成线并平行的电动马达形式，可被用于驱动火车，一部分马达在火车头上，其余的部分在轨道上）；直线电动机

induction coil　a coil for producing a high voltage from a low-voltage source　感应线圈

magnetic field　the lines of force surrounding a permanent magnet or a moving charged particle　磁场

Earnshaw’s theorem　厄恩肖定理

permanent magnet　a magnet that retains its magnetism after being removed from a magnetic field　永磁铁，永久磁铁

particle accelerator　a machine used for research in nuclear physics which can make particles that are smaller than atoms move very fast　粒子加速器；质点加速器

escape velocity　theminimum velocity that a body must have in order to escape from the gravitational field of the earth or other celestial body　[物][天]逃逸速度，脱离速度

Notes

1.In physics， magnetism is one of the phenomena by which materials exert an attractive or repulsive force on other materials.

2.In physics， friction is the non-conservative resistive force that occurs when two surfaces travel along each other when forced together.It causes physical deformation and heat buildup.

3. Superconductivity is a phenomenon occurring in certain materials at low temperatures，characterised by the complete absence of electrical resistance and the damping of the interior magnetic field（the Meissner effect.）Superconductivity is a quantum-mechanical phenomenon that is different from perfect conductivity.

4. Electrodynamic Suspension （EDS）is one method that can be used for maglev trains.Superconductor electromagnets on the train generate a magnetic field.Propulsion coils on the guideway are used to exert a force on these magnets，and make the train move forwards.

5. Electromagnetic Suspension （EMS）in Maglev trains is a system whereby a powered electromagnet maintains a train at a constant distance from a track.It is distinguished from electrodynamic suspension（EDS）.

6. Inductrack is a completely passive magnetic levitation train system，using only unpowered loops of wire in the track and permanent magnets（in Halbach arrays）on the train to achieve levitation.Inductrack was invented by Lawrence Livermore National Laboratory physicist Richard E.Post.The only power required is to push the train forward，with increasing levitation force generated as the velocity of the train increases over the loops of wire.

7.A linear motor is essentially an electric motor that has had its stator“unrolled”so that instead of producing a torque（rotation），it produces a linear force along its length.

Many designs have been put forward for linear motors，falling into two major categories，low-acceleration and high-acceleration linear motors.Low-acceleration linear motors are suitable for maglev trains and other ground-based transportation applications.High-acceleration linear motors are normally quite short，and are designed to accelerate an object up to a very high speed and then release the object.They are usually used for studies of hypervelocity collisions，as weapons，or as mass drivers for spacecraft propulsion.

When a linear motor is used to accelerate beams of ions or subatomic particles，it is called a particle accelerator.The design is usually rather different as the particles move close to the speed of light and are usually electrically charged.

8.An induction coil （archaically known as a Ruhmkorff coil ）is a type of disruptive discharge coil.These coils are passive electrical device used to produce high voltage pulses from a low voltage DC supply.

9.In physics，a magnetic field is an entity produced by moving electric charges（electric currents）that exerts a force on other moving charges.（The quantum-mechanical spin of a particle produces magnetic fields and is acted on by them as though it were a current；this accounts for the fields produced by“permanent”ferromagnets.）A magnetic field is a vector field：it associates with every point in space a vector that may vary in time.The direction of the field is the equilibrium direction of a compass needle placed in the field.

10. Earnshaw’s theorem states that a collection of point charges cannot be maintained in an equilibrium configuration solely by the electrostatic interaction of the charges.This was first stated by Samuel Earnshaw in 1842.It is usually referenced to magnetic fields，but originally applied to electrostatic fields，and，in fact，applied to any classical inverse-square law force or combination of forces（such as magnetic，electric，and gravitational fields）.

11. Diamagnetism is a very weak form of magnetism that is only exhibited in the presence of an external magnetic field.It is the result of changes in the orbital motion of electrons due to the external magnetic field.The induced magnetic moment is very small and in a direction opposite to that of the applied field.When placed between the poles of a strong electromagnet，diamagnetic materials are attracted towards regions where the magnetic field is weak.Diamagnetism is found in all materials；however，because it is so weak it can only be observed in materials that do not exhibit other forms of magnetism.Also，diamagnetism is found in elements with paired electrons.Oxygen was once thought to be diamagnetic，but a new revised molecular orbital（MO）model confirmed oxygen’s paramagnetic nature.

12.An electromagnet is a type of magnet in which the magnetic field is produced by a flow of electric current.The magnetic field disappears when the current ceases.

13.The Lawrence Livermore National Laboratory（LLNL） is a United States Department of Energy national laboratory，managed by the University of California，in Livermore，California.Along with Los Alamos National Laboratory，it is one of the USA’s two laboratories whose mission has included the design of nuclear weapons.

14.A Halbach array is a special arrangement of permanent magnets which augments the magnetic field on one side of the device while cancelling the field to near zero on the other side.In the diagram，the magnetic field is enhanced on the bottom side and cancelled on the top side（a one-sided flux）.The pattern（on the front face；left，up，right，down）of permanent magnets can be continued indefinitely and have the same effect.It is roughly similar to many horseshoe magnets placed adjacent to each other，with alternating polarity.

The effect was discovered by Mallinson in 1973，and described as a“magnetic curiosity”although he recognized that this magnetization pattern could make significant improvements in magnetic tape technology.Mallinson showed that any magnetization pattern in which the x and y components of the magnetization are π/2 out of phase will cause a“one-sided”flux，the simplest being sin（kx），-cos（kx）.It was later（and independently）discovered by Halbach，who used the arrangement in particle accelerators.

This device is the fundamental principle behind the Inductrack maglev system，a levitating train that requires no power to levitate；power is only used to create forward motion.The Halbach arrays repel buried loops of wire after they have been accelerated to speed，lifting the train.Halbach arrays were originally developed in a cylindrical shape，known as the Halbach cylinder.

15.A particle accelerator is a device that uses electric and/or magnetic fields to propel electrically charged particles to high speeds.

16.The National Aeronautics and Space Administration（NASA） ，which was established in 1958，is the agency responsible for the public space program of the United States of America.It is also responsible for long-term civilian and military aerospace research.

17.In physics，for a given gravitational field and a given position，the escape velocity is theminimum speed an object without propulsion，at that position，needs to have to move away indefinitely from the source of the field，as opposed to falling back or staying in an orbit within a bounded distance from the source.The object is assumed to be influenced by no forces except the gravitational field；in particular there is no propulsion，as by a rocket，there is no friction，as between the object and the Earth’s atmosphere（these conditions correspond to freefall）and there is no gravitational radiation.This definition may need modification for the practical problem of two or more sources in some cases.In any case，the object is assumed to be a point with a mass that is negligible compared with that of the source of the field，usually an excellent approximation.It is commonly described as the speed needed to“break free”from a gravitational field.

18.In electricity， current refers to electric current，which is the flow of electric charge.Lightning is an example of an electric current，as is the solar wind，the source of the polar aurora.Probably the most familiar form of electric current is the flow of conduction electrons in a metallic wire.This is how utility companies deliver electricity.In electronics，electric current is most often the flow of electrons through conductors and devices such as resistors，but it is also the flow of ions inside a battery or the flow of holes within a semiconductor.

19. Mach number （Ma）（pronounced as“mack”in International English or“mock”in the American English）is defined as a ratio of speed to the speed of sound in the medium in case.The Mach number is commonly used both with objects travelling at high speed in a fluid，and with high-speed fluid flows inside channels such as nozzles，diffusers or wind tunnels.As it is defined as a ratio of two speeds，it is a dimensionless number.At standard sea level conditions，Mach 1 is 1，225km/h in the atmosphere.

Exercises

Ⅰ.Answer the following questions according to the text.

1.What’s the definition of maglev？

2.How much time does Shanghai maglev take people to the airport？

3.How many primary types of maglev technology are there？What are they？

4.What does inductrack use for stabilization？

5.What are the pros and cons of EMS？

6.What are the pros and cons of EDS？

7.What are the pros and cons of inductrack？

Ⅱ.Choose the best answer to each question.

1.A maglev is a type of train that is___________in the air above a single track.

A.suspended　　B.propelled　　C.operated　　D.travelled

2.The train can be levitated by the repulsive force of___________poles.

A.opposite　　B.unlike　　C.like　　D.dislike

3.A new，perhaps less-expensive，system is called___________.

A.EDS　　B.inductral　　C.EMS　D.none of the above

4.___________has narrow gap between track and vehicle.

A.EMS　　B.EDS　　C.Inductral　　D.none of the above

5.The top speed of Shanghai Maglev is___________km/h.

A.250　　B.431　　C.268　　D.650

Ⅲ.Translate the following sentences into Chinese.

1.A maglev is a type of train that is suspended in the air above a single track，and propelled using the repulsive and attractive forces of magnetism.

2.Japan and Germany are active in maglev research，producing several different approaches and designs.

3.The effect of a powerful magnetic field on the human body is largely unknown.

4.Some space agencies are researching the use of maglev systems to launch spacecraft.

5.Inductrack was originally developed as a magnetic motor and bearing for a flywheel to store power.

Ⅳ.Summary writing.

In no more than 80 words give an account of this passage.Use your own words as far as possible. oDfRSg1rchBqvqt64kG77iy4ck0B9RA7UHqzu2iYt5aKKp9sz1EWDW+nPQumNmYs