For those wanting to understand a little more; an article on ammeters:
https://en.wikipedia.org/wiki/Ammeter
The hangup over voltage drop measurements is a valid one. Many guys know how to measure voltage, but fewer understand how to measure amperage (current) or Ohms (resistance). And mechanics use voltage drop measurements all the time (or they should be) to diagnose amperage problems. The analog multimeter is measuring voltage by inferring its potential by the amount of current flowing through its meter movement and some resistors in series with the movement. (A digital meter works differently, with voltage playing a much larger role. It has no electromagnetic movement that demands current to generate a magnetic field.) With voltage drop measurements we can identify the problem when the starter doesn't want to to crank too well; instead of starting to throw money at the problem by immediately blaming the battery, we can take measurements across the battery connections, ground connections, master and starter contactor terminals, and so forth, and find the real reason the starter is lazy. Very often it's not the battery, but some other component in the circuit that is presenting resistance where none belongs. Bad contactors and corroded connections are most common.
But we must remember that, with the analog multimeter and the airplane's steam-gauge ammeter, we're making mechanical meter movement with a magnetic device, which relies on amperage to make it move. If we say that the shunt is generating a voltage drop and that is what the meter is measuring, we're not really wrong but we're muddying the water by bringing voltage into it. Amperage is flowing through the shunt, which forces a much smaller amperage through a meter movement that has a high resistance, which uses it to move the needle. And that's what a voltmeter does when we take a voltage drop measurement across a corroded connection: that connection becomes a shunt that should have practically zero resistance but has more than zero, and it's forcing some current through the "voltmeter" that uses a milliammeter to measure that current and read it out to you on a scale calibrated in volts. The "voltage" is inferred from the current measurement.
Ohm's Law says that E=IxR, or voltage is equal to current times resistance. If we know two of the values we can calculate the third. E/R=I, and E/I=R. When we talk about
power, we need E and I: ExI=P, or Watts.
For my students that had a hard time with this, I talked about the garden hose. The pressure that forces the water through the hose is like voltage. Without it, nothing happens. The rate of water flowing in the hose is like amperage (current); more flow gets more work done. It waters the garden faster, It puts the fire out faster. It gets your sister wetter faster. The sprinkler or a nozzle is like resistance; it represents a load on the system. Pretty hard to get any real work done without at least some resistance. All heaters or lights or motors or radios present resistance, and their resistance is what limits the current flow. If your sister kinks the garden hose so you can't squirt her, she is putting a massive resistance into the system and the water stops, or nearly so.
If we raise the water pressure, we get a higher rate of flow. If we raise voltage, we get more current flow. If we raise the hose's resistance, we get a lower rate of flow, same as a raised resistance in an electrical circuit. If the hose breaks, the resistance gets really low and all that water goes to ground
. A broken wire stops electrical flow; electrons don't run out all over everything. A wire that touches ground (airframe) is a short, a low-resistance connection that results in a massive current flow that either trip a breaker or pops a fuse or causes an electrical fire. Fuses and breakers are there to protect the wire, not the component it's feeding. The component has resistance that limits the current flow through it; it's that wire that will light up if the current finds some alternate, low-resistance path to ground.
The positive battery cable has no fuse or breaker in its circuit to the bus behind the instrument panel. That's why the master contactor is located close to the battery: so we can shut the master off if we smell smoke. A fuse or breaker would need to be close to the battery, where we couldn't reach it if it popped. And it would be a massive device, to handle the large current flow demanded by the starter. So mechanics should be paying close attention to the condition of those cables and connections so that shorts don't develop.
