[vcf-midatlantic] Apple IIe Voltmeter

[Systems Glitch]

> But I found my Simpson 260 and other analog meters, put LESS load 
> because they generally use 50uA movements (hence 20,000 ohms per volt, 
> do the math).

Yep, one reason I don't have any non-Simpson analog meters anymore!

> As to "figuring out the accuracy".....create a 10mA 
> circuit from an accurately-measured battery and an accurately-measured 
> resistor. Say 5V and 500 ohms, or whatever - the DVM can measure those 
> fine, other resistors give other current sources, and math gives you the 
> exact current. Many batteries can generally supply 10mA stable for 
> minutes and hours or more, and your DVM can monitor their voltage anyway.

Exactly my plan. I just needed something I knew was accurate on the voltage measurement. That's the problem with two instruments you're uncertain of -- "a man with a watch always knows the time; a man with two watches is never sure!"

> Why? Given the "digital is better" bias these days, only a few people 
> call out the Simpson 260 as a valued instrument - Dave McGuire for 
> instance, in that email list we used to use.

I prefer an analog meter for troubleshooting -- like you said, easier to detect an "is it off" condition, especially with intermittent circuits. And, the loading of the analog meter can be an advantage in many situations. My Simpson 260 is always on the bench, and I keep the Simpson 160 in my toolkit since it's just the tiny version of the 260.

> Jonathan, if you determine a cause for your analog meter's apparent 
> errors, let me know.

I opened it up this afternoon, it looks like the shunt for the 100 mA scale is wire (nichrome?) wrapped on a phenolic bobbin, typical of old small-value precision resistors. It's sleeved in some sort of plastic tube which appears to have been clear originally, but is now green over the section of wire. I'm guessing you're right and corrosion has thrown the value off.

Thanks,
Jonathan