Electromagnetic Testing

Oersted discovered the presence of a magnetic field around a current carrying conductor and
observed magnetic field developed in a perpendicular plane to the direction of current flow in a
wire. Ampere observed that equal and opposite currents flowing in adjacent conductors cancelled
this magnetic effect. Ampere's observation is used in differential coil applications and to
manufacture non inductive precision resistor. Faraday's first experiments investigated induced
currents by the relative motion of magnet and a coil (Figure 1.2). Faraday's major contribution
was the discovery of electromagnetic induction. His work can be summarized by the example
shown in Figure 1.3.
A coil "A" is connected to a battery through a switch, "S", A second coil, B, connected to a
voltmeter is near by. When switch S is closed it produces a current in coil A in the direction
shown (a). A momentary current is also induced in coil in direction (b) opposite to the current
flow in coil A. If S is now opened, a momentary current will appear in coil B having the direction
of (c). In each case current flows in coil B only while the current in coil A is changing.
Charlie Chong/ Fion Zhang