across the terminals when not in a circuit to prevent
damage to the movement. This will dampen any
movement of the needle since the coil within the
magnetic field can act as a generator when the meter is
If a meter is not sensitive enough to measure very low
currents, an operational amplifier (op-amp) with a very
high input impedance can be used to amplify the current
so that it can be measured. The Texas Instruments’
OPA129 has a Difet input bias current of 100
femtoamperes (1E- 13) and can boost very low currents to
the point where they can be displayed on an analog
meter. An interesting approach can be viewed at
Customizing the Meter Scale
There are several ways of making a custom scale.
Years ago, I wrote a program in the Hewlett-Packard
Printer Control Language (PCL) to produce various meter
scales. I have also produced scales using Microsoft Paint.
The process using both of these methods was tedious.
Today, however, there is a much better “Meter” program
available on the Internet from www.TonesSoftware.com.
One of the advantages of an analog meter is when
measuring a parameter that is changing fairly rapidly in
real time — as in the measurement of plate current in a
high frequency amplifier that is being modulated with
voice in Single Sideband (SSB) or Morse code (CW).
The analog meter gives an immediate (average)
indication of the change in current where an abnormal
reading can indicate a problem. An attempt to duplicate
this effect has been used in some of the RF linear
amplifiers made by Alpha and others using a string of
Light Emitting Diodes (LEDs), but there is something
magical about watching the analog meter in real time.
Where to Purchase Sensitive
Manufacturers of analog panel meters are becoming
quite scarce, but new meters are still available online. The
panel meter shown in Figure 14 has a full-scale reading of
50 microamperes and can be configured to measure most
parameters. They are available from China for about $7
(and they pay the shipping!). I currently have several of
these meters in various circuits and they work great!
One of the classes I took in my first year of college in
the early 1960s was taught by a retired Navy man on the
various types, configurations, and uses of analog meters. (I
Most meters could be bought for one or two dollars.
This was a memorable time for us hobbyists and ham-radio types. We put meters on everything just so the
equipment would look impressive in a 19” rack. (They
were cheap, so why not!)
Using an analog meter to make any measurement is
only limited by one’s imagination. Plus, it’s fun to think up
new uses for them. A measurement can be esoteric
enough that it might take the casual observer some time
to figure out what they are looking at. How about
measuring the height of an antenna on a motorized tower
in Femto-Light-Years (FLYs); see Figure 15. (Nah! That
might be a bit too weird!)
Femto = 10E - 15.
One light year = 5,869,713,600,000 miles
A FLY turns out to be almost exactly 31 feet! Happy
(analog) metering! NV
FIGURE 14. A 50 microampere panel meter.
October 2017 49
FIGURE 15. A reading of 1.9 FLY = a height of
58. 9 feet.