84. Declination. For convenience, we shall represent the true N and S, at the place where you are experimenting, by the full line, N S, in [Fig. 23]. The dotted line shows the direction taken by the compass-needle. The angle, A, between them, is called the angle of variation or the declination. This angle is not the same for all places; and, in fact, it changes slowly at any given place; so it becomes necessary to construct magnetic maps for the use of mariners and others.
EXPERIMENT 45. To study the lines of force above and below a bar magnet placed horizontally.
Apparatus. A bar magnet, B M (No. 21); compass, O C (No. 18).
85. Directions. (A) Lay B M upon the table and place O C upon its center. Note the position of the compass-needle.
(B) Slide O C along from one end of B M to the other, and study the effect upon its needle. Do lines of force curve over B M as well as around its sides, as shown in [Exp. 31]?
(C) Place O C upon the table. Hold B M horizontally above O C, and move O C back and forth under B M. Does the needle remain horizontal, or does it show that lines of force pass under B M on their way from its N to its S pole?
