or
| HP = | 2ml | , |
| d³ |
the direction being parallel to AB.
And
| I = | d³HP | = | d³HE | tan θ. |
| v | v |
(46)
 | |
| Fig. 7. | Fig. 8. |
(3) In the third position the test rod is placed vertically with one of its poles at the level of the magnetometer needle, and in the line drawn perpendicularly to the undeflected needle from its centre of suspension. The arrangement is shown in fig. 8, where AB is the vertical rod and M indicates the position of the magnetometer needle, which is supposed to be perpendicular to the plane of the paper. Denoting the distance AM by d1, BM by d2, and AB by l, we have for the force at M due to the magnetism of the rod
| HP = | m | − horizontal component of | m |
| d1² | d2² |
| = m ( | 1 | − | d1 | ). |
| d1² | d2³ |