Part Two Magnetic Compass & Gyro Compass

Passage One Magnetic Compass

Learning Objectives: basic principles of Magnetic Compass (MC) and its advantages over Gyro Compass (GC) composing parts of a MC limitations of a MC in direction-finding ability Permanent and induced magnetism of a MC and the correction for same 3 components of a total permanent magnetic field effect

Vocabulary compass bowl 罗经碗 binnacle / 'binəkl/罗经柜 binnacle stand 罗经台 gimbal n. /'dʒimbəl/ [pl] (=gimbalring)(使罗盘等平衡的)平衡环; 平衡架 chamber n. 容器 flinder’s bar 垂直软铁棒 soft iron 软铁 cardinal points 基点方位 intercardinal points 象限方位点 deviation /,di:vi'eiʃən/n. 自差 Gimbal system to keep card level when boat heels and pitches Each ship is made of Steel and Iron. It has its own magnetic field that can effect changes in the ship’s magnetic compass. Electronics can also cause deviation. It changes with the ship’s heading.

semicircular deviation (BC) correcting device半圆自差矫正器 quadrantal /'kwɔdrəntl/ adj. 象限的 quadrantal deviation (D) 象限自差 heeling n. 倾斜 heeling error ( H) 倾斜误差 sluggish adj. 反应迟钝的，反应灵敏度低的sluggishness n. 反应迟钝 align /ə'lain/vt. place in a line or arrange so as to be parallel or straight使成一线 align oneself with 调准，校直 e.g. Could you align this table with those? 你能把这张桌子同那些桌子排成一行吗？ The north-seeking gyro compass functions by continuously aligning itself with the meridian using the earth’s rotation and gravitational effects.陀螺罗经利用地球的旋转和地心引力作用寻找到子午线并不断与其校直来实现真方向的测向。

permanent magnetism 永久磁性 induced magnetism 感应磁性 demagnetize /di:'mæɡnitaiz /vt. 除去磁性 deflection n. 偏向，偏转 tilt vt. 使倾斜vi. 倾斜 sine n. 正弦 cosine n. 余弦 sine and cosine curves 正弦和余弦曲线 reverse vt. 反转，颠倒，倒退

The magnetic compass is one of the oldest items of navigational equipment. It gets its source of power from the earth’s magnetic field. Despite the rising importance and great convenience of the gyrocompass, the magnetic compass still retains its importance because of its simplicity and reliability. The magnetic compass will remain operative even when a ship is subjected to the following: electrical power failure, fire, collision, grounding or other hazards. 即使船舶受到诸如断电、火灾、碰撞、搁浅或其它险情影响，磁罗经仍然可用。be subject to 受制于；以…为条件 e.g. The king muse be subject to the law. 王子犯法，与庶民同罪。

Fluid filled damping system to impede rapid card movement Magnetic compasses are composed of a compass bowl and a binnacle stand. The bowl is a bowl-shaped container of nonmagnetic material (brass) which serves to contain the magnetic elements, a reference mark, and the fluid. It is supported in the gimbals within the binnacle. The compass card is an aluminum disc, graduated in degrees from 0° to 360° . It also shows cardinal and intercardinal points. It can work satisfactorily against an inclination of within 10'. The magnetic needle (made of MK magnetic steel) sticks positively to the float, which is an aluminum, air-filled chamber in the center of the compass card, and its magnetism never diminishes due to ageing. The binnacle stand incorporates the deviation correcting devices provided for by the regulations, such as the semicircular deviation (B.C), quadrantal deviation (D), heeling error (H) correcting devices and the flinder’s bar (F). （用MK磁钢制成的）磁针紧紧贴着浮起物，浮起物是一个位于罗盘中央、内充空气的铝制小容器。磁针的磁性不会因老化而减小。 stick to：贴着，粘着，相当于attach to。 due to ageing: 由于老化的原因，due to相当于because of，owing to，as a result of。 Fluid filled damping system to impede rapid card movement

The following characteristics of the magnetic compass limit its direction-finding ability: 1) Sensitive to any magnetic disturbance 2) Useless at the magnetic poles and is sluggish and unreliable in areas near the poles 3) Deviation changes as a ship’s magnetic properties change 4) The magnetic properties also change with changes in the ship’s structure or magnetic cargo 5) Deviation changes with heading 6) Does not point to true north. Because the magnetic north pole and the true North Pole are not located at the same point, the magnetic compass does not seek true north. The magnetic compass aligns itself with the magnetic meridian. The angular difference between the magnetic meridian and the true meridian is called “variation” because it varies at different points on the earth’s surface. 磁子午圈与真子午圈的角度差叫做“磁差”，因为磁差会随着船只所处地表位置的不同而不同。

The magnetic compass must be corrected for the vessel’s permanent and induced magnetism so that its operation approximates that of a completely nonmagnetic vessel. Magnetism that is present only when the material is under the influence of an external field is called “induced magnetism.” Permanent magnetism is the magnetism remaining for long periods without appreciable reduction unless the material is subjected to a demagnetizing force. Ship’s magnetic conditions create magnetic compass deviations and sectors of sluggishness and unsteadiness. Deviation is defined as deflection right or left of the magnetic meridian. Adjusting the compass consists of arranging magnetic and soft iron correctors about the binnacle so that their effects are equal and opposite to the effects of the magnetic material in the ship. 磁罗经必须要根据船舶的永久磁性和感应磁性来矫正，这样，它才能接近无磁船磁罗经的效果。 approximate：本句中，不是形容词，表近似的，而是动词，表示接近。that of…：that代词代替了前文当中的 “operation”; its代表magnetic compass’s 矫正罗经时，要调整罗经柜处的磁铁矫正器和软铁棒，从而使它们的效力与船体磁场影响相等并相反。 magnetic and soft iron correctors：并列的定语修饰名词correctors，是两种矫正装置。 永久磁性是长久存在且没有明显减弱的一种磁性，除非材料受到除磁力量的影响。 appreciable reduction相当于obvious reduction, reduction that can be easily seen. be subjected to 遭受，受到影响，to为介词。

The total permanent magnetic field effect at the compass may be broken into three components, mutually 90° apart. The vertical permanent component tilts the compass card, and, when the ship rolls or pitches, causes oscillating deflections of the card. Oscillation effects which accompany roll are maximum on north and south compass headings, and those which accompany pitch are maximum on east and west compass headings. The horizontal B and C components of permanent magnetism cause varying deviations of the compass as the ship swings in heading on an even keel. Plotting these deviations against compass heading yields the sine and cosine curves. These deviation curves are called semicircular curves because they reverse direction by 180°. 随着船首向在水平方向上左右摇摆，永久磁性的水平B和C组件会使罗经产生各种不同的自差。 on an even keel: 不倾斜的，平稳的

Questions？ Homework: Read the passage again, and keep in mind the key words and expressions; Preview Passage Two on Gyro Compass, and look up the new words in it.