XPost: sci.electronics.basics   
   From: dj@delorie.com   
      
   "Thomas G. Marshall"    
   writes:   
   > If I connect an ammeter (from my handy multimeter) to a battery,   
   > what is it measuring?  The amount of amps allowed to pass through   
   > the wires, etc. of the multimeter?   
      
   If you connect an ideal battery to an ideal ammeter, it will read   
   infinite amps.   
      
   However, the battery has internal resistance, wires have some   
   resistance, the ammeter has internal resistance.   
      
   So if you connect a real battery to a real ammeter, you get the   
   battery's voltage divided by the actual total resistance   
   (Rbatt+Rwire+Rmeter).   
      
   A battery which can only provide 1mA would have a high internal   
   resistance, perhaps 3000 ohms.  A car battery can provide hundreds of   
   amps at 12v, closer to 0.1 ohms.   
      
   > If it depends entirely on load, then why wouldn't the ammeter always   
   > measure the same regardless of what I connect it to?   
      
   In normal circuits, the resistance of the battery is negligible.   
   You're making up abnormal circuits, so it isn't.  Plus, amps depends   
   on the volts as well; for a *given* battery, the amps depends on the   
   load.  For *all* batteries, it also depends on the voltage of the   
   load.  V=IR, as long as you account for all those tiny internal   
   resistances.   
      
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