Geoscience Reference
In-Depth Information
if we think of placing our right hand around the current-carrying wire, with our thumb in the direc-
tion of current flow, then the direction of the magnetic field is in the same direction as our fingers
point. We often call the directional lines that represent the magnetic field,
lines of force
. Maxwell's
contribution was to modify Ampere's law to include time-varying EM fields (a changing electric
field) along with conduction currents.
Ampere looked at induction from the point of view of the magnetic field caused by the flow of
electric current and only perceived the phenomenon in terms of his concept of action-at-a-distance.
Conversely, Michael Faraday turned the problem around and considered it from the perspective of
the magnetic field. He developed the concept of lines of magnetic induction. Faraday's concept is
the basis for our way of visualizing the magnetic field in terms of lines-of-force and flux-density.
Faraday's law states that a moving magnetic field can change current flow in a conductor in a man-
ner that is the converse of Ampere's law. The simplest experiment to demonstrate Faraday's law is
to move a magnet through a loop of wire, as shown in Figure 6.2a. If a current-flow meter (Amp
meter) is attached to the loop, then the current flow that is measured is proportional to how fast
the magnet is moved through the loop. The current in the loop is called the induced current, and the
moving magnet that is inducing the current is called an induced electromotive force. An analogous
experiment utilizes two loops, as shown in Figure 6.2b, with current flowing through the loop on the
Right Loop
(transmitter)
LOOP
Left Loop
(receiver)
Current
Source
Amp
meter
Volt
meter
(a) Moving bar magnet inducing
an electric current in a loop
(b) Current flow in right loop causes a magnetic field
which induces current flow in the left loop
Receiver: induced current
flow through wire coil
Source: current
flow through wire
coil
SECONDARY MAGNETIC FIELD
from “receiver” coil
Resulting inducing PRIMARY
MAGNETIC FIELD in space
(c) Current source causes magnetic field inducing a current flow in receiver coil
fIGURe 6.2
Principles of induction: (a) a magnet moving through a loop causes current flow in the coil;
(b) current generated in one wire loop causes a magnetic field that induces current flow in a second coil; and
(c) the combination of Ampere's and Faraday's laws as used in geophysical electromagnetic equipment. (Parts
(a) and (b) modified from Halliday, D., and Resnick, R., 1960,
Physics
(parts I and II), John Wiley & Sons,
New York. With permission.)
