Galvanometer

[David Nebenzahl]

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?

 

[David Nebenzahl]

With a standard alkaline cell and starting with a *very high* resistance
(5 megohms, say), drop the resistance until you get a decent deflection.
Then measure the resistance and the voltage, and do the math.

Figure it out for myself? What a concept!

So, OK, I did as you suggested. Actually, the thing was a *lot* less
sensitive than you warned; I ended up using 2 AA cells (2.95 volts
according to 2 DMMs) through less than 10K to get a decent deflection
from the meter.

So here's my work, which you can check:

The needle went left to 18.

I = V (2.95) / R (7.45K)
= 0.0004A (rounded) = 0.4mA

Dividing this by 18 shows that each division on the meter is 0.02mA.

So I hooked the thing up to one channel's mike input on my Sony D6
Walkman (see posts rehantom powering from this device above) through
an appropriate load resistor, and the meter went to 15, which tells me
that it was supplying 0.3mA. (Did the same thing even without any load
resistor.)

Not much power. (Even less than Eeyore had predicted this unit would be
able to supply.) Can't use that to power my microphone as I'd hoped. Oh,
well.

 

[David Nebenzahl]

Most meter *movements* will only read directly small currents or voltages.
To allow them to read higher values you use an appropriate resistor - in
series to read a higher voltage than the FSD of the movement or in
parallel to read a higher current.

Yes, I know all that.

So the exact same movement can be supplied with a variety of scales for
different applications - and may or may not include this resistor, which
is called a shunt for current and multiplier for voltage. If it includes
this resistor the scale is usually marked with the true units.

No shunt resistor. See my post above: turns out each division is 0.02mA
(0.6mA full scale). Meter is labeled "DC GALVANOMETER", by the way. I'm
thinking it was a piece of lab equipment, or maybe for students to learn
to use stuff like Wheatstone bridges, etc.

30-0-30 was common for the ammeter on older cars.

How do you get from "galvanometer" to automobile ammeter???

Helpfulness of your post: 0.2 (dimensionless units).

 

[GregS]

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.

Michael

I also though of a sensitive current or sensitive votmeter. I actually though of a static vane
type, but there is also the iron vane galvanometer. The key was the center scale 0.

greg

 

[David Nebenzahl]

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.

Well, this one is definitely marked as such (at the factory), and was
made by what was at the time one of the largest manufacturers of meters
in the U.S., Weston Electrical Instrument Corp. Make of that what you will.

 

[GregS]

Well, this one is definitely marked as such (at the factory), and was
made by what was at the time one of the largest manufacturers of meters
in the U.S., Weston Electrical Instrument Corp. Make of that what you will.

Here is one I have....

http://zekfrivolous.com/misc/meter.JPG

 

[David Nebenzahl]

Here is one I have....

http://zekfrivolous.com/misc/meter.JPG

Except ... it's not labeled as a galvanometer. Nice piece, though; much
nicer looking than my utilitarian black crackle job.

 

[Franc Zabkar]

With a standard alkaline cell and starting with a *very high* resistance
(5 megohms, say), drop the resistance until you get a decent deflection.
Then measure the resistance and the voltage, and do the math.

Isaac

A quicker way to do it might be to use two DMMs. Set the first to
measure resistance and the second to measure current. Start with the
highest resistance scale. DMM1 will supply a test current to the other
two meters.

DMM1 (ohms) DMM2 (amps) Weston Galvanometer

--> I --> I
o---------o-- A --o---------o-- M --o--|
| |
current |
source |
| |
o--------------------------------------|
I <--

My DMM outputs the following test currents:

diode - 0.93mA
200R - 167uA
2K - 133uA
20K - 42uA
200K - 5uA
2M - 0.5uA

When testing on the diode range, the OP could set DMM2 for the highest
burden, ie the 2M scale. In that way the voltage to the galvanometer
would be reduced by 200mV (?). Alternatively he could insert a series
resistor or potentiometer.

If only one DMM is available, then replace DMM1 with a battery and
resistor and use your DMM to measure the current, otherwise you will
need to account for the burden of the galvanometer when doing your
Ohm's Law calculations.

- Franc Zabkar

 

[att]

A quicker way to do it might be to use two DMMs. Set the first to
measure resistance and the second to measure current. Start with the
highest resistance scale. DMM1 will supply a test current to the other
two meters.

DMM1 (ohms) DMM2 (amps) Weston Galvanometer

--> I --> I
o---------o-- A --o---------o-- M --o--|
| |
current |
source |
| |
o--------------------------------------|
I <--

My DMM outputs the following test currents:

diode - 0.93mA
200R - 167uA
2K - 133uA
20K - 42uA
200K - 5uA
2M - 0.5uA

When testing on the diode range, the OP could set DMM2 for the highest
burden, ie the 2M scale. In that way the voltage to the galvanometer
would be reduced by 200mV (?). Alternatively he could insert a series
resistor or potentiometer.

If only one DMM is available, then replace DMM1 with a battery and
resistor and use your DMM to measure the current, otherwise you will
need to account for the burden of the galvanometer when doing your
Ohm's Law calculations.

- Franc Zabkar

Every galvanometer I've seen in the past 60 years read in microamps

 

[att]

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.

 

[Franc Zabkar]

Every galvanometer I've seen in the past 60 years read in microamps

The OP's measurements suggest that the meter's fsd is +/- 600uA, not
+/- 30uA. Therefore it probably has a current shunt.

- Franc Zabkar