I need to measure the voltage drop across a short piece of wire (about .25 to .30 inches long). The wire (a fusible link) is permanently attached to a battery which is permanently connected to a system. There is no way to insert an ammeter. No, measuring the voltage of the battery will not satisfy my problem because each battery is one of many in parallel. Would an electrometer (Keithley or whatever) be a useful device to measure the voltage drop across the short piece of wire? I haven't used and electrometer for decades!
Electronic – How to measure voltage drop across a wire
voltage
Related Solutions
There is one thing that has been missed out from your 'classic resistor example' - the internal resistance of the battery.
If you simply measure the battery voltage with a voltmeter you get a higher reading due to the fact that there is no (or very little) voltage dropped across the internal battery resistance.
The voltage measured across a load resistor (or bulb) OR the terminals of the battery will be smaller by an amount equal to I * R(int).
The connecting wire has very little resistance and so does not contribute to a significant voltage drop between the terminals of the battery and the resistor so the voltages at these two places are, in practice, the same.
V1, R1, and LAMP1 are connected in parallel in your circuit. That means that the voltage across R1 is equal to V1 or 12 volts. Likewise, the voltage across LAMP1 is also equal to V1 or 12 volts. Thus the current through R1 is 12 volts/100 ohms or 120 milliamperes. The current through LAMP1 is 12 volts/100 ohms or also 120 milliamperes. Note that the current through R1 is the same as the current through LAMP1 because their resistance is the same, not because they are connected in parallel. In a practical circuit, the interconnecting wires would have some small resistance (small compared to 100 ohms) so that the voltage across R1 would be slightly less than V1. The voltage across LAMP1 would also be slightly less than R1 due to the voltage drop in the wires between R1 and LAMP1.
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Best Answer
Normally I would run a constant current from an external power supply and measure the voltage drop on the wire. If my current is 1A, the voltage indicates the exact resistance of the wire, so 4.6mV, for example, would mean 4.6mΩ.
If you can't measure below 0.1mV (100μV), you can increase the current to 10A (if your supply is capable) and that way measure a 10 times smaller resistance. In this case, 4.6mV would indicate a 0.46mΩ resistance.