Why is it necessary to connect ammeter in series and voltmeter in parallel?

Question

Please justify well.

Answer 1

Because you are measuring two different forms of electrical action.

Power = watts = Amps X Voltage.
Power = the work done, a car's horsepower is the same thing 1 horsepower = 78 watts.

Voltage is the electrical potential (what it can do) of a circuit.
Amperage is the current (electrical pressure) of a circuit, what it is doing.

A voltmeter checks the battery or how much power is going into the circuit. No matter where you check the voltage on a simple circuit (say a light bulb or motor only in series) the voltage will be the same.

An ammeter tells you what is going on inside the circuit. In a simple circuit the amperage can remain constant, but if you have a complex circuit then the amperage will change across the circuit. Unless you have a part the steps down the voltage (a transformer can do this) then the voltage will be constant.

You wall outlet generates the same amount of voltage all the time. This voltage varies from region to region, but the target is 110 to 120 volts. If you plug a little night-light in then you will have 120 volts, but you won't need a lot of wattage so you will be using a low amperage, that's the only variable. If you plug a 500-watt work light into that same circuit of 120 volts then you can only get those extra watts by drawing more current or more amperage. If you use a voltmeter in this circuit, no matter how you use it you will get the same reading, even if you turn on more lights in the same circuit. In this circuit the amperage will remain the same, until you start changing the circuit by adding more or less load to the circuit. At that point your ammeter will get different readings depending what the load is and where the ammeter is in relation to the loads.

A simple voltmeter experiment has a simple magnetic meter with a large resister in series with it shorted across the poles of power. You want to know what that power source can give you. It will always give you the same potential no matter what amperage it is pulling. So you put a heavy load (the large resistor) across the circuit with the meter in the same circuit. It doesn't matter what the value of the resister is, as long as it protects the meter from burning out. If you have a 1 gig ohm, a 1 mega ohm, or a 1 kilo ohm in that circuit then you will get the same reading on the meter.

A simple amperage meter is that same magnetic meter with NO resister in the circuit and the meter is included as part of the circuit. The amp meter is trying to measure how well this circuit can perform with the power source attached. If you short the amp meter across the poles then you will get the maximum current of the power source running through the circuit. That doesn't tell you anything, and it is a quick way to kill a power source. Most power sources have a maximum current that the can give out. In batteries the amps drop with age and use, but the voltage remains constant, until the battery is too weak to perform.

You want the ammeter in the same circuit that you are trying to read, and you don't want to add or subtract anything to that circuit which might change your readings so you want the lowest resistance possible. As the reading of the amp meter changes then you know that the power of the circuit is changing, remember the voltage is constant.

A normal ammeter requires that you cut the circuit and connect the two cut ends to the positive and negative leads of the meter. You only do this on ONE section or one line of the circuit. If you do it anywhere else then you create a short circuit, your load (like a light or a motor) won't operate and your reading is useless.

The second type of ammeter uses an induction coil that encircles one line of the circuit. This can be done without compromising the circuit. So it has a minimum impact on the circuit, and you want to change the circuit as little as possible so you know what the load is doing to it.

With a voltmeter, you don't care if the load is a tiny light bulb or 30 big screen TVs. In fact when you measure the voltage you ignore the load and short the circuit so the load doesn't receive any current. The only reason you want a large resistor in the circuit is to protect that meter.

To put it simply a 9-volt battery gives you nine volts until it dies. It doesn’t drop to 6, 4, or 3 volts; it is an all or nothing thing. Another words the potential, what the battery can do, remains constant until it dies. So you can just measure it from pole to pole. The ammeter measures how much current is being used. When you want to find out how much power is being used in a circuit then the voltage is fixed, and the amperage is the only variable. If you have a series of loads in a circuit then the amperage is going to vary with each load that is operating, so you need to put the meter directly into that circuit to understand what is going on and you want to change that circuit as little as possible.

Voltmeters use brut force to find out what is going on. No matter what the load is the voltage is constant, it doesn’t change. It is there at the power source and throughout the basic and simple circuit.

Ammeters use a meter to detect the subtle changes going on the in the circuit. These things change as the load varies.

Answer 2

Both are basically galvanometers that sense the flow of charge. An ammeter has to capture the flow of current, all the current, so you have to put it in series. Voltmeter measures the potential difference. So we connect it across the lines between which we want to measure the voltage. It has a large resistance, the current flowing through it is a small value and is proportional to the voltage.

Answer 3

think of electricity as a liquid for a moment. an ammeter measures flow, hence it is put in the line of flow. A volt-meter measures pressure(voltage equates to this) but to do this it still reguires a complete circut so it must be connected to both poles. I know it sounds strange and over simplified but thats it. hope it helped, Mark

Answer 4

The current is dependent on the load and not the meter. The meter either needs to measure a small voltage drop across a small value resister or measure the strength of a magnetic field on a wire with the load current flowing through it. Either way to access the current it has to flow through the meter and not by the meter.