Page:A Treatise on Electricity and Magnetism - Volume 2.djvu/146

Next, reverse the magnetism of the needle and determine its new magnetic moment $M^{\prime }$, by observing a new deflexion, the tangent of which is $D^{\prime }$,

Then place it on its bearings and take two readings, $\theta _{3}$ and $\theta _{4}$, in which $\theta _{3}$ is nearly $\pi +i$, and $\theta _{4}$ nearly $-i$,

whence, as before,

adding (8),

whence we find the dip

where $D$ and $D^{\prime }$ are the tangents of the deflexions produced by the needle in its first and second magnetizations respectively.

In taking observations with the dip circle the vertical axis is carefully adjusted so that the plane bearings upon which the axis of the magnet rests are horizontal in every azimuth. The magnet being magnetized so that the end $A$ dips,, is placed with its axis on the plane bearings, and observations are taken with the plane of the circle in the magnetic meridian, and with the graduated side of the circle east. Each end of the magnet is observed by means of reading microscopes carried on an arm which moves concentric with the dip circle. The cross wires of the microscope are made to coincide with the image of a mark on the magnet, and the position of the arm is then read off on the dip circle by means of a vernier.

We thus obtain an observation of the end $A$ and another of the end $B$ when the graduations are east. It is necessary to observe both ends in order to eliminate any error arising from the axle of the magnet not being concentric with the dip circle.

The graduated side is then turned west, and two more observations are made.

The magnet is then turned round so that the ends of the axle are reversed, and four more observations are made looking at the other side of the magnet.