Page:A Treatise on Electricity and Magnetism - Volume 2.djvu/149

obtained in one hour, and the average error of a single observation is a fraction of a minute of arc.

It is proposed that the dip needle in the Cambridge Physical Laboratory shall be observed by means of a double image instrument, consisting of two totally reflecting prisms placed as in Fig. 19 and mounted on a vertical graduated circle, so that the plane of reflexion may be turned round a horizontal axis nearly coinciding with the prolongation of the axis of the suspended dip-needle. The needle is viewed by means of a telescope placed behind the prisms, and the two ends of the needle are seen together as in Fig. 20. By turning the prisms about the axis of the vertical circle, the images of two lines drawn on the needle may be made to coincide. The inclination of the needle is thus determined from the reading of the vertical circle.

Fig. 19.	Fig. 20.

The total intensity $I$ of the magnetic force in the line of dip may be deduced as follows from the times of vibration $T_{1}$, $T_{2}$, $T_{3}$ $T_{4}$ in the four positions already described,

The values of $M$ and $M^{\prime }$ must be found by the method of deflexion and vibration formerly described, and $A$ is the moment of inertia of the magnet about its axle.

The observations with a magnet suspended by a fibre are so much more accurate that it is usual to deduce the total force from the horizontal force from the equation

where $I$ is the total force, $H$ the horizontal force, and $\theta$ the dip.

464.] The process of determining the dip being a tedious one, is not suitable for determining the continuous variation of the magnetic