In article
,
"att" wrote:

"David Nebenzahl" wrote in message
.com...
On 4/13/2009 9:39 AM msg spake thus:

David Nebenzahl wrote:

I have an old galvanometer (nice Weston in a slant-front black crackle
case). Its scale reads 30-0-30. What units are these? Or are they just
relative values for comparison?

When you said 'galvanometer', I immediately expected you to be talking
about a 'mirror galvanometer': a large sensitive laboratory instrument
rather than an ordinary moving coil meter, which of course is
also a 'galvanometer' but is not commonly (at least in the US) labeled
as such.
That instrument was called a "ballistic galvanometer" and was intended to
measure
very small currents for a very short duration, the mirror would deflect with
the current and return very slowly so you could grab a reading on the
current.

A "ballistic galvanometer" is rather a different beast from a normal
galvo -- for one thing, the suspension has essentially no restoring
force (and that is why the return is so slow; not just to make it easier
to read). A normal galvo deflects to the angle where the *continuous*
force from the current is matched by the restoring spring force of the
suspension.

"Ballistic" refers to the fact that the galvo is intended to accept a
brief pulse of current and then respond "ballistically" -- integrating
the pulse -- by producing a deflection proportional, essentially, to the
total number of electrons that just flowed through its coil. In fact,
the pulse should be short enough so that it is completely over before
the deflection begins. In general, just about *any* continuous current
you put through a ballistic galvo will cause it to deflect all the way
to the stops (because the restoring force is so small).

Isaac