[By an M.I.N.A.]

Progress, Volume II, Issue 11, 2 September 1907, Page 409

[By an M.I.N.A.]

In the process of building a ship the vibration caused b}^ the riveting and other operations turns the ship into a permanent magnet, the strength of the magnetism and the direction of its polarity depending on the direction of the ship during the building. This property has its effect on the compasses, and is the reason for the various operations of compass correction which have to be carried oiit on every new ship, magnets being placed around the compass to counteract the mag netism of the ship. Captain Bartling, R.N.R., has carried out some most interesting expetiments while the s.s. Thveringen was building at Bremen, and he has given the results in a short paper before the North-East Coast Institution of Engineers and Shipbuilders, read on Friday, the 24th May. The ship was built on the slip with the bow pointing S.r>3.3 c E.. and observations were taken at intervals to see how the strength and polarity of the magnetism varied. As the process of riveting was carried out so the magnetism increased, and then a bridge structure was put bodily into place. As it happened the effect of this was opposite to that of the ship. The effect of launching a ship from the adjoining slip was tested, and found to be very slight. When the ship herself was launched she was placed nearly as possible in the opposite direction to that of building, viz., N.32°W., and it was found as time went on that the original magnetism was more and more neutralised, and she practically required no compass correction at all. She went to sea withovt a compensating magnet for the standard compass, and with only one compensating magnet for the steering compass. Such a state of things is highly desirable, because the compass is more sensitive to the earth's influence, and it is a point which shipbuilders might well bear in mind that if it can be done without inconvenience, it is a good thing to finish the ship afloat in the opposite direction to which she was built. In a sister ship to the Thuain^en the ship was berthed in a direction 5.31.,V'E., and it was found that seven compensating needles had to be put in the compass. The effect on the compass of the magnetism of the ship and the compensating magnets is to diminish the directive force, and if this be H on shore it will only be KH on board,

X being generally about 0.8. In a space like a conning tower, where the lines of magnetic force are drawn through the walls of the tower, X only equals 0.3, and in such a compass is so sluggish as to be practically useless. In recent warships the compass is placed in the lower steering position and the steersman in the conning tower looks down a telescope, or has the movements of the compass reflected up to him. Captain Bartling makes the suggestion that horizontal material in the immediate neighbourhood of the compass should be made of non-magnetic nickel steel and he has done this in a large vessel now building, and from experiments he has made he hopes to arrive at a value of X of 0.9 to 095 in the above expression for the directive force on board. A navigating officer depends so completely on his compasses for the correctness of his observations and the safety of his ship that owners and shipbuilders should do everything in their power to make the magnetic properties of the ship such that the compasses should be as sensitive and correct as possible. The suggestion above as to the bearing of a ship after launching could, in many cases, be easily carried out by builders if their attention was drawn to its desirability. — The Mariner and Engineering Record.