The magnetic field astern of a ship

The magnetisation of a ship may be divided into a permanent and an induced part. If the induced part is a linear function of the field components and the permanent part is independent of them, the disturbance of the total force by the ship contains a term independent of magnetic heading and terms proportional to the sine and cosine of the heading and of twice the heading, the sine terms being less than a tenth of the cosine terms.

The variation with distance astern of the ship is similar to that due to a pole near the bow and one near the stern plus a line of vertical dipoles.

These results are verified by experiment. The disturbance of the field by Discovery II and by Sarsia is less than 10 γ at a distance of 2 ship's lengths astern. It is believed that this result will hold for most ships. A method for the determination of the principal coefficients and their variation with distance is described.