The Ship's Magnetic Compass

Question 1

what did a magnetic compass typically comprise of?

Answer 1

• simple lightweight compass card
• needle magnets suspended below
• mounted within a gimballed compass bowl

Question 2

why was friction at the pivot of a magnetic compass kept to a minimum?

Answer 2

the extreme light-weight of the compass card assembly

Question 3

why did the card remain horizontally stable?

Answer 3

the centre of gravity of the card assembly was well below the pivot

Question 4

how was the low centre of gravity of the card assembly achieved?

Answer 4

by suspending the magnetic needles below the card on silk threads

Question 5

where was the standard compass normally positioned on a ship?

Answer 5

on the monkey island

Question 6

why was the standard compass normally positioned on the monkey island on a ship?

Answer 6

it’s above the major concentration of ferromagnetic materials that make up the ship’s structure

Question 7

what sort of compass was the steering compass?

Answer 7

a liquid compass

Question 8

describe the makeup of the liquid compass:

Answer 8

• translucent compass card mounted on a float
• a ring magnet is secured to the float
• well below the pivot - to lower the centre of gravity of card assembly + keep card essentially horizontal in all magnetic latitudes

Question 9

(in a liquid compass) why is the compass bowl filled with a liquid of a specific gravity?

Answer 9

supports most of the weight of the card assembly + reduces friction on the bearings

Question 10

(in a liquid compass) why is the viscosity of the liquid specifically selected?

Answer 10

to provide good damping of the card movement

Question 11

what is the construction of the bowl in a liquid compass?

Answer 11

• brass with a clear glass top
• frosted glass bottom to allow the card to be illuminated from below
• expansion ring/chamber
• filling cap at the side

Question 12

why is the card considerably smaller than the bowl in a liquid compass?

Answer 12

keep it clear of any turbulence created round the edges of the bowl as the ship swings

Question 13

what is the bowl mounted on within the binnacle?

Answer 13

gimbals

Question 14

what is the binnacle used for?

Answer 14

• protecting the compass
• provides fittings for various devices used to adjust the compass
• low voltage lighting system to illuminate the compass card

Question 15

what is the binnacle constructed of?

Answer 15

• non-magnetic materials

Question 16

why is the binnacle positioned on the centreline of of the ship?

Answer 16

ensure uniform distribution of steel on either side

Question 17

what is the Azimuth mirror or ring used for?

Answer 17

taking compass bearings

Question 18

where is an Azimuth mirror fitted?

Answer 18

over the rim of a compass bowl about which it is free to rotate

Question 19

what shape is the Azimuth mirror?

Answer 19

triangular prism

Question 20

how do you change the rotation of the Azimuth mirror when taking bearings of celestial objects?

Answer 20

rotated ‘arrow up’

Question 21

how do you change the rotation of the Azimuth mirror when taking bearings of terrestrial objects?

Answer 21

rotated ‘arrow down’

Question 22

how do you use an Azimuth Mirror to get bearings of a celestial object?

Answer 22

• observer aligns the reflected image of the celestial body with direct image of the compass card

Question 23

how do you use an Azimuth Mirror to get bearings of a terrestrial object?

Answer 23

direct image of the terrestrial body is aligned with the reflected image of the compass card

Question 24

what is a pelorus used for?

Answer 24

taking bearings of objects when compass bearings cannot be taken

Question 25

what is a pelorus in its simplest form?

Answer 25

a sighting device mounted on a 360* graduated plate with 0* aligned with the ship’s fore + aft line