On 2006-08-02, jasen <jasen@xxxxxxxxxxx> wrote:
>>>> I recently tested a 9V alkaline battery by measuring its open-circuit
>>>> voltage (9.0 V) and then measuring it with a car headlight lamp (R = 1
>>>> Ohm) across the terminals (4.0 V). The lamp lit up brightly and got
>>>> warm, but from the significant voltage drop I conclude that the
>>>> battery is basically dead. Correct?
> If you've got lots of spare automobile parts there,
> perhaps "3W" panel indicator lamp would be more realistic
> or a 22 ohm resistor.
Perhaps I should mention that the only reason I did this test was that
I happened to notice a used 9V battery and a headlamp lying near each
other on my workbench, and curiosity drew me in!
>> Right. I measured the lamp's resistance with an ohmmeter, which of
>> course puts very little current through it.
>> But I took the measurements by clipping the voltmeter (actually it's
>> the same meter) leads onto the battery terminals, reading the
>> open-circuit voltage, then pressing the lamp's terminals against the
>> battery terminals (the spacing was convenient --- that's where I got
>> the idea from) and immediately reading the loaded voltage (before the
>> lamp heated up).
I wrote those two paragraphs in the wrong order. I measured the
voltage drop first and then (after letting the lamp cool) measured its
If I'd seen the 1 Ohm value first, I wouldn't have used it for the