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Unit Construction. A cross section and circuit representation of
the M 4 is shown schematelly in figure 152. Two mu-metal cylinders lead the
magnetic field to two pieces which are adjacent to the needle. The needle is
a cylindrical drum magnet mounted on a horizontal axis with its poles
normally fixed at right angles to the axis of the pole pieces. The drum
magnet consists of a quantity of steel powder within a bakelite matrix. In
con-structing the magnet, the steel powder is pressed into the matrix, and,
in that form, is magnetized. The top pole piece has tow coils (R and H-1)
with adjusting resistors (W 1 and W 2) mounted next to them. Mounted on the
lower pole piece is a single coil (H 2). A small d-c motor is mounted on the
pole piece. This motor has a permanent magnetic field and a reduction gear
train. Since the motor is rigidly fixed on the pole piece, it maintains a
constant orientation with respect to the needle magnet. The magnetic effect
of the motor is compensated for in latitude adjustment. |
