Picking back up on the garden hose ****ogy: A volt meter measures pressure. It's like feeling how hard it is to keep your thumb over the end of a hose. The volt meter feels the pressure, just like our thumbs. Only instead of telling us how many thumbs it takes, it tells us how many volts. In the example above, we have two hoses Y'd together at a faucet. One hose is running into a bucket so we can measure gallons (our amp meter). The other hose has our thumb over it so we can feel the pressure (our volt meter). Right now, there's not much pressure on our thumb because the end of the bucket-hose is completely open. You could probably hold the second hose shut with just your pinky finger. There's very little pressure in either hose. Now imagine we put a nozzle on the bucket-hose and adjusted it so it's only half open. The pressure in the thumb-hose would also rise with the pressure in the bucket-hose because they're both connected back at the Y. We wouldn't be able to hold it shut with our pinky any more. We'd have to use our thumb. The more we close the nozzle on the bucket-hose, the higher the pressure is on our thumb, until it takes two thumbs to hold it. The pressure is always exactly the same in both hoses. If you think about it, there was never any water flowing through the thumb-hose. We had our thumbs over it. We would have felt just as much pressure on our thumbs if the thumb-hose were smaller, because no water actually had to move. We could have even used a very tiny hose like the ones on the back of an oil pressure gauge, if we wanted to. That's why we can use a small wire to measure volts. We're just measuring the pressure, not the quan***y. There's almost no electricity flowing through the wire to the volt meter.
That has to do with the design differences between a generator and an alternator. A generator will usually maintain voltage if it's starting to fail or is being pushed beyond it's capacity; it will be it's current output that drops off. So with a generator, measuring current (amps) is a better indicator of system operation. The opposite is true of an alternator; it will drop voltage output before current when failing or exceeding it's output, so a voltmeter will be the better indicator.
So as I understand things NOBODY in his right mind would install an ammeter in an automobile today. Why Henry Ford installed them in every Model T and A with a starter is a reason lost to history. Bob
I never understood why ammeters were ever used. The condition of the charging system is better indicated by the voltage than amperage. As has been described, the ammeter is just a voltmeter measuring the voltage drop over a specific resistance. As such it can determine how much current is flowing and which direction it is flowing, into the battery or out of the battery. A voltmeter can tell if the current is flowing into or out of the battery. If the voltage is higher than battery voltage, current will flow into the battery. If voltage is lower than battery voltage, current is flowing out of the battery. If the voltage gets too high, it can overcharge and damage the battery. If it gets too low the battery has been discharged too much. An ammeter should return to the center zero position after the battery is charged but it can be at zero without charging the battery if the charging system is only producing just enough to supply the vehicle current demand. Maybe the voltmeters available in the early years weren't sensitive enough to easily determine the difference in a 2 volt range. It was easier to determine the difference between positive and negative voltage to determine the direction of flow so the ammeter was used. Of course, whatever is used first becomes "traditional" so changing it later becomes a problem. We all know how that works.
Perhaps the batteries used 80 years ago were less even in performance. That could make a voltage reading less useful.
It is possible to have volts but no amps. I'm sure the change to voltmeters was for cost savings in manufacturing as well as safety. Sent from my VS987 using The H.A.M.B. mobile app
Absolutely, that's an open circuit, hook a wire to the battery positive, leave the other end unhooked and it will show battery voltage but zero current because it has no where to flow Sent from my iPhone using The H.A.M.B. mobile app
In other words, put a volt meter in your rig. You can wire it to any 12 V source that is switched with the ignition.
"........and some manufacturers used an external shunt that was mounted on the firewall, thus a low voltage signal was fed to the “Amp” meter that was actually a volt meter calibrated in amps......or was this already stated.... Sent from my iPad using The H.A.M.B. mobile app
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