Tangent Galvanometer: 8 Turns on an 11-Inch Wooden Ring with a Pocket Compass (Boy Mechanic, 1913)

"Any deviation from the dimensions will cause errors in the results obtained by its use."

From 1/2 in wood cut a ring 10-1/2 in outside and 9 in inside. Glue on each side a 1/4 in ring, 9 in inside and 11 in outside, to form a 1/4 in channel round the edge (or turn it from solid). Use brass screws and brads only.

Wind 8 turns of No. 16 insulated magnet wire in the channel and tie the ends.

Cut a round base 11 in across with a slot 1 in wide and 6-5/16 in long in the middle. Stand the ring in it with its inner surface level with the top of the base, held by a small strip screwed to the base. Fit two binding posts and connect the coil ends.

Fix two strips 1/4 x 5/8 x 11 in across the ring with their top edges exactly through its centre, and set a pocket compass about 1-1/4 in across in them, needle centre at the ring centre, zero half-way between the strips. Coat everything with brown shellac.

Keep iron, steel and magnets away. On a level table turn it until the needle lies in the plane of the coil at zero. Connect the circuit; 45 degrees means 1/2 ampere; use the table for other angles. His table holds near Chicago and east of it north of the Ohio; multiply by 1.3 south of the Ohio and east of the Mississippi.

From **The Boy Mechanic, Book 1** (Popular Mechanics, 1913), pp. 150-151 (checked on the archive.org scan boymechanic0000unse_n5a8). His table equals 0.5 x tan(angle); the field-strength note and modern enamelled wire are ours.