In article ,
David Nebenzahl wrote:

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

In article ,
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?

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.

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