Galvanometer

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

On 4/12/2009 8:08 PM isw spake thus:

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 re:phantom 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.

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.

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.

On 4/13/2009 1:26 AM Dave Plowman (News) spake thus:

Yes, I know all that.

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.

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

Helpfulness of your post: 0.2 (dimensionless units).

Then why the post? You have all the information to work it out for yourself.

Similar meters are - or were - on sale in most hobby electronics shops.

FFS, a 'galvanometer' *is* an ammeter. Or is it yet another term you want to mean whatever you think it should? And 30-0-30 is exactly how older car ammeters were marked. No need for 'amps' as that's all it could be. And external ones for garage use (on cars not so fitted) were to be found.

You really are a charmer.

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

2. greg

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

On 4/13/2009 9:39 AM msg spake thus:

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....

On 4/13/2009 11:19 AM GregS spake thus:

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

On Sun, 12 Apr 2009 20:08:37 -0700, isw put finger to keyboard and composed:

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

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

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.

On Mon, 13 Apr 2009 21:54:51 GMT, "att" put finger to keyboard and composed:

The OP's measurements suggest that the meter's fsd is +/- 600uA, not

- Franc Zabkar

That sort of input is intended to power an electret condenser mic (with possibly a FET follower). It basically needs a polarizing voltage at nearly zero current.

Isaac

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

On Tue, 14 Apr 2009 22:11:42 -0700, isw wrote about ballistic galvanometers.

Thanks for the refresher! I'd forgotten about ballistic galvo's. Your description of minimal restoring force reminds me of the very distinctive traditional Megger meter movements; their hairsprings (three of them) were tweaked for no net restoring force.

Btw, "galvanometer" is a term from more than a century ago; iirc, it refers to Luigi Galvani, of voltaic pile and jumping frog's legs, etc.

Regards, nb