1. The problem statement, all variables and given/known data Describe and account for two constructional differences between a moving-coil galvanometer used to measure current and the ballistic form of the instrument. An electromagnet has plane-parallel pole faces. Give details of an experiment, using search coil and ballistic galvanometer of known sensitivity, to determine the variation in the magnitude of the magnetic flux density (magnetic induction) along a line parallel to the pole faces and mid-way between them. INdicate in qualitative terms the variation you would expect to get. A coil of 100 turns each of area 2.0 * 10-3 m2 has a resistance of 12 Ω. It lies in a horizontal plane in a vertical magnetic flux density of 3.0 * 10-3 Wb m-2. What charge circulates through the coil of its ends are short-circuited and the coil is rotated through 180 ° about a diametral axis? Answer: 100 μC. 2. The attempt at a solution We need to find charge Q. Q = N A B / R. We don't know B. B = (d Φ / d A) * A = 3 * 10-3 * 2 * 10-3 = 6 * 10-6 T. So Q = 100 * 2 * 10-3 * 6 * 10-6 / 12 = 1 * 10-7 C. The correct answer is 100 * 10-6 C or 10-4 or 0.00 01. While my answer is 10-7 or 0.00 00 00 1. What's wrong?