- Analog Meter
instruments are the simplest of the instruments available for
eddy current inspections. They are used for crack detection, corrosion
inspection, or conductivity testing. These types of instruments
contain a simple bridge circuit, which compares a balancing load
to that measured on the test specimen. If any changes in the test
specimen occur which deviate from normal you will see a movement
on the instruments meter.
Analog meters such as the D'Arsonval design pictured
in the applet below, must "rectify" the AC into DC.
This is most easily accomplished through the use of devices called
diodes. Without going into elaborate detail over how and why diodes
work as they do, remember that they each act like a one-way valve
for electrons to flow. They act as a conductor for one polarity
and an insulator for another. Arranged in a bridge, four diodes
will serve to steer AC through the meter movement in a constant
An analog meter can easily measure just a few microamperes of
current and is well suited for use in balancing bridges.
the equations within the applet, calculate appropriate values
for C and R2 for a set of probe values. Then balance the bridge
using your calculated values. The analog meter should swing close
to the left end if its scale indicates little or no current across
the bridge. Across the bridge should be minimized (straight line).