Standard Magnetic Comes 3f60

STANDARD MAGNETIC COMES Question 1 (a) Label clearly and completely the diagram below

A- Night attachment light

I- Erector lens

B- Water tight plug & socket

J-

Field lens & intermediate image

C- Junction box

K-

Projector lens

D- Caulked watertight

L-

Lamp to light card from below

E- Viewing Mirror

M-

Transparent com card

F- Grey glass screen & final image

N-

Condenser lens

G- 1/8 thick rubber gasket

O-

Projector lamp

H- 6 ½ diameter hole in deck

(b) Describe how to remove an air bubble from a liquid com bowl. This can be achieved by removing the thumbscrews that hold the magnetic com into the gimbals, taking the bowl down to a manageable height and turning it so that the screw is located vertically at the top. The bubble (s) is then carefully manoeuvred underneath the screw, the screw removed and using a syringe to fill up the bowl with the correct liquid (industrial alcohol or distilled water). (c) What is/are the function(s) of the liquid in the com? The liquid in the com serves a number of purposes: a. It dampens the oscillation of the com card, b. It lightens the weight of the card on the pivot, therefore, reducing the wear and tear (lubricate) of the material, which the pivot is made of. (d) (i) Define “heeling magnet” Heeling Magnet is a permanent magnet placed vertically in a tube under the center of a marine magnetic com, to correct for heeling error. (ii) Explain briefly the purpose of the heeling magnet and how this purpose is achieved The heeling magnet is used to compensate for the change in the deviation of a magnetic com when a vessel heels, due to the change in the position of the magnetic influences of the vessel relative to the earth’s magnetic field and to the com. The heeling magnet is the only corrector which corrects for both permanent and induced effects. Therefore, it must be adjusted occasionally for changes in ship’s latitude. However, any movement of the heeling magnet will require readjustment of other correctors.

Question 2

(a)

Explain the theory of magnetism as applied to ferromagnetic materials and state the difference(s) between permanent and induced magnetism. Ferromagnetic materials have a large and positive susceptibility to an external magnetic field. They exhibit a strong attraction to magnetic fields and are able to retain their magnetic properties after the external field has been removed. Ferromagnetic materials have some unpaired electrons so their atoms have a net magnetic moment. They get their strong magnetic properties due to the presence of magnetic domains. In these domains, large numbers of atom's moments (1012 to 1015) are aligned parallel so that the magnetic force within the domain is strong. When a ferromagnetic material is in the un-magnetized state, the domains are nearly randomly organized and the net magnetic field for the part as a whole is zero. When a magnetizing force is applied, the domains become aligned to produce a strong magnetic field within the part. Iron, nickel, and cobalt are examples of ferromagnetic materials. Ferromagnetic materials become magnetized when the magnetic domains within the material are aligned. This can be done by placing the material in a strong external magnetic field or by ing electrical current through the material. Some or all of the domains can become aligned. The more domains that are aligned together, the stronger the magnetic field in the material. When all of the domains are aligned, the material is said to be magnetically saturated. When a material is magnetically saturated, no additional amount of external magnetization force will cause an increase in its internal level of magnetization. Permanent magnets are magnets that are formed from hard iron. They are formed by either placing the hard iron in a magnetic field or by ing electricity through the hard iron. As a result the hard iron becomes magnetized and will retain this magnetism after being removed from the inducing magnetic field, that is, it becomes permanently magnetized. On the other hand, induced magnetism is that which is experienced when a magnetic material is placed in a magnetic field, it is experienced only as long as the material remains in the magnetic field, but the induced magnetism is lost as soon as it is taken out of the magnetic field.

(b) Define the following : (i)

Agonic Lines - These are imaginary lines on the earth’s surface connecting points where the magnetic declination (variation) is zero. The agonic line is a line of longitude on which a com will show true north, since where magnetic declination is zero, coincides with geographic north.

(ii)

Isoclinic Lines - These are imaginary lines connecting points on the earth's surface having equal magnetic dip.

(iii)

Angle of Dip – is the angle that a magnetic needle makes with the horizontal at any point on the Earth’s surface. This angle is considered positive in the Northern Hemisphere where the red end (North Pole) of a magnet points below the horizon and the opposite occurs in the Southern Hemisphere.

(iv)

Permeability – is the degree of magnetization of a material that responds linearly to an applied magnetic field.

(c) (i)

Define “deviation” and explain briefly the conditions that may cause deviation to change name and magnitude. Deviation is the deflection of the com needle from the magnetic meridian due to disturbances in the surrounding magnetic field; it is the difference between the magnetic heading and the heading shown on a magnetic com. Deviation changes with an alteration of course. When a vessel alters course, the induced magnetism in the vessel also changes. Also, deviations can be caused if magnetic materials are placed near the com, if there is a change of magnetic latitude and if there is a magnetic storm.

(ii)

What is the actual cause of variation and why does it vary from place to place. Variation results from the fact that the geographical poles are not in coincidence with the magnetic poles. Variation is the difference between the North Magnetic Pole and the North Geographical Pole. Variation

varies because of irregularities and abnormalities in the structure of the Earth’s Magnetic field, which causes variation to change even with latitude. Another reason is that the magnetic poles are not stationary, but they tend to migrate resulting in changes in variation even at the same place. (iii)

Describe the method of swinging the com to obtain a deviation table, by observing two (2) objects in transit. By this method, establish a transit line of known bearing. Record the bearing and the heading of the vessel. The vessel is then swung through 360 degrees, observing the bearing of the transit every 15 degrees of heading change and comparing it with the com. Any discrepancy in these readings gives the deviation for that particular heading. From these figures a deviation table can be composed.

Question 3 (a) What is the purpose of the Magnetic Com Record Book The purpose of the book is to record all the activities of the magnetic com on board. It should have a record of the observations of the deviation of the com on different courses; any adjustments made to any part of the com, regular overhaul and if there are any suspected damages to the com. It also serves the purpose of comparing the magnetic com to the gyrocom. (b) Describe how an entry is made and the frequency at which this is done. The Magnetic Com Record book is part of the official record of a ship and is maintained as an adjunct to the Deck Log aboard every vessel in commission. It is a complete history of each magnetic com on board Each volume of the Magnetic Com Record contains a sufficient number of Com Check Log forms for 3 months' continuous entries, based upon half-hourly observations. Observations may be made at shorter than half-hourly periods if desired. In those cases where the ship is not operating continuously, the book will be usable for a more extended period.

Provision is made in this book for accommodating the record of both the standard and steering magnetic comes. the comes must be compared every onehalf hour and at each course change. Note: There is an exception; if a ship is in a formation and changing course frequently, or the ship is alongside another ship, each course change does not need to be recorded. Use the following statement in the Remarks column: "Steering various courses while alongside ." A comparison must still be made every one-half hour.

Question 4 (a) What is the essential difference between the operation of the Magnetic Com and its counterparts the gyro com The essential difference is that the magnetic com depends on the earth’s magnetic field for its operation, while the gyrocom on the other hand depends on the properties of a gyroscope, the earth’s rotation and gravity. (b) List the criteria of the liquid in the magnetic Com (i) To dampen the oscillations of the com card. (ii) To reduce the wear and tear of the pivot point by lightening the weight of the com card. (iii) It is an anti-freeze mixture, which ensures that the com can be used in cold weather. Question 5 List the considerations to be taken into when choosing a siting location for the Magnetic Com onboard the vessel i.

The com should be mounted at least three (3) feet from large magnetized masses and at least two feet three inches (2’ 3”) from fire extinguishers, electrical circuits, another com and repeaters of electronic instruments.

ii.

Care should be taken to ensure that the lubber line is placed on or parallel to the fore and aft line of the vessel. If from whatever reason it is impossible to place it

on the center line, it can be mounted either to the right or left ensuring that it remains parallel to the center line. iii.

There must be a 360 degrees view of the horizon.

iv.

The com must be easily accessible to persons using it and allow the helmsman an ease of the eyes from observing the water to observing the com.

v.

It should be kept away from any metal portable objects that can cause serious deviation.

Question 6 Explain how the Com card is kept practically horizontal in all Latitude. The card is maintained nearly horizontal by arranging the system in such a manner that the centre of gravity of the card and needles is below the pivot. The weight of the card is contained largely in the aluminium ring at its circumference and it therefore possesses a large moment of inertia in comparison with its weight. This fact, in association with an appropriate magnetic moment for the system of needles, produces a stable card and no external damping is necessary. The long period of vibration achieved ensures that the card is not caused to oscillate in time with the much shorter natural vibrations of the ship.

Question 7 Give a detailed description of the use and purpose of the single axis fluxgate element in a magnetic com. A magnetic com can be converted into a repeating com by the addition of a flux gate assembly, which will produce output pulses to drive remote repeaters. A flux gate element is effectively a magnetometer which is used to detect both the magnitude and direction of a magnetic field. The basic flux gate element consists of two thin wires of mumetal or permalloy, each wire contained in a glass tube, around which is wound a coil, two such assemblies are used and are mounted side by side and parallel to each other. The two coils are connected in series so that their magnetic fields are in opposition when a low frequency AC is applied to them, mumetal is used because of its property to be magnetically saturated at very low levels of magnetic flux. A secondary coil is then wound around the whole assembly; this provides a mutually induced E.M.F. for the output voltage. If the current in coil a changes then the magnetic flux it causes will also change either in value or direction and a mutually induced E.M.F is produced. If a magnet is placed at right angles to the element the effects will cancel out each other, if it is placed parallel then its strength is increased. When the element is

used in conjunction with the magnetic com then it utilizes the north seeking property of a permanent magnet. A flux gate is mounted on a rotatable platform mounted in the com bowl below the com card, the core elements of the flux gate will therefore come under the influence of the permanent magnetic field produced by the com pointer.