A wire at right angles to a uniform magnetic field experiences a force. It is calculated with the equation: force (N) = magnetic flux density (T) × current (A) × length (m), or F = B × I × L. The magnetic flux density, B, tells you how strong the field is, and it is measured in tesla, symbol T. The length is the length of wire that is inside the field.
Example: a wire in a field of 0.20 T carries 3.0 A, and 0.15 m of it is in the field. The force is 0.20 × 3.0 × 0.15 = 0.090 N. Doubling the current doubles the force, so a current of 6.0 A would give 0.18 N. Halving the length makes the force half as big.
The equation can be rearranged: B = F ÷ (I × L), I = F ÷ (B × L) and L = F ÷ (B × I). Always use SI units, so a length given in centimetres must be converted to metres before you calculate.