but a lot of instruments use what are called taut-band suspension. The meter
coil or vane and pointer are suspended on a flat ribbon running along the axis
of rotation and fastened at the ends. The band acts as both support for the
moving mechanism and as the restoring spring. It is essentially a torsion
spring.
The advantage of this system is that it eliminates the bearings along with
the friction and hysterisis they cause. Taut-band meters have very good
repeatability and can be very accurately calibrated. Hewlett-Packard began to
use taut-band meters in their instruments along around the early 1960's and
they became common around that time. Taut-band meters do have zero-adjustments
but they may be internal.
The damping of any meter is a matter of the back EMF of the meter. No meter
running open circuit will be well damped. It may be that the testing circuit
does not have low enough resistance to damp the meter. If you rotate the meter
open circuit the pointer wil move around a lot. If you short the terminals it
will barely move at all.
Richard Knoppow
Los Angeles, CA, USA
Hi Dick,
in taut band meters the taut band replaces the torsion springs. But the
Hallicrafters meter has a rigid axis (not taut band), and has neither torsion
springs nor a moving coil, so it must work on a different principle.
Thank for the advice and 73
Tony I0JX
the Hallicrafters