EXPERIMENT 31. To study the space around a magnet, in which pieces of iron become temporary magnets by induction.
Apparatus. A bar magnet, B M (No. 21); a compass (No. 18); a sheet of stiff paper about 1 ft. (30 cm.) square, with a center line, C L, drawn parallel to one of its sides ([Fig. 16½]), and with another line, E W, drawn perpendicular to C L. (See Apparatus Book, Chap. VI., for various ways of making home-made permanent magnets.)
61. Directions. (A) Lay the paper upon the table, and place the compass over the center of the line, C L, previously drawn.
(B) Place the eye directly over the compass-needle, then turn the paper until the line is N and S; that is, until the line is parallel to the length of the needle. Pin the paper to the table to hold its center line N and S.
(C) Place B M upon the paper, as shown ([Fig. 16½]), its N pole to the north, and its center at the cross line, E W.
(D) Slowly move the compass entirely around and near B M, and note the various positions taken by the needle. Note especially the way in which its N pole points. This is to get a general idea of the action of the needle.
(E) Place the compass in the position marked 1, which is on E W, about 1 in. from the line, C L. Press the wooden support down firmly upon the paper to show, by the dent made in the paper by the pin-head, the exact place on the paper that is under the center of the compass-needle. Before removing the compass from this position, look down upon it again, and make a dot on the paper with a pencil directly under each end of the needle. Remove the compass, and draw a line through the dent and the two dots just made. This will show a plan of the exact position of the needle.