A truck drives onto a loop detector and increases the downward component of the magnetic field within the loop from 1.2 x 10^- 5 T to the larger value B in 0.38 s. The detector is circular, has a radius of 0.67 m, and consists of three loops of wire. What is B, given that the induced emf is 8.1 x 10^- 4 V ? A truck drives onto a loop detector and increases the downward component of the magnetic field within the loop from 1.2 x 10^- 5 T to the larger value B in 0.38 s. The detector is circular, has a radius of 0.67 m, and consists of three loops of wire. What is B, given that the induced emf is 8.1x10^- 4 V?