Ammeters,Voltmeters,Ohmmeters

This page is created by jlukose6

This page will describe certain tools that are used in measuring various values inside of a circuit. Using these tools can help you calculate current, voltage, power and resistance across various circuits.

Ammeters

What is an Ammeter

An Ammeter is a measuring device used to measure the electric current in a circuit. It can be used in both series and parallel circuits. This is plausible through the very negligible resistance that the Ammeter introduces to the circuit. It allows us to measure the current running through a circuit element very closely. An Ammeter measures current in Amperes(A) named after Andre Marie Ampere.

Connecting Ammeters

To properly use Ammeters, they must be inserted into a circuit in series with the element that you want to measure. You bring the current into the + socket and out of the - socket. If a positive number is read then the conventional current is flowing into the + socket. Therefore, a negative number is read when conventional current flows into the - socket.

Ammeter in Parallel?

One rule of Ammeters is to have a low resistance so it doesn't alter the current through the circuit and the element that is being measured. So,what happens if you connect the Ammeter in parallel?

This would create an extremely low resistance path("short circuit"). All of the current would then pass through the Ammeter and the reading would be a measurement of the maximum current that the batteries(voltage source) can provide.

Voltmeters

What is a Voltmeter

A voltmeter is a measuring device used to measure the potential difference across two points in an electrical circuit. A Voltmeter is created by adding a series resistor (with resistance R) to an Ammeter and attaching this across an element. There is a measured current of I and the potential difference across the resistor is Va – Vb = RI. The ammeter is then relabeled and now measures Volts.

"Voltage is the measure of electrical “push” ready to motivate electrons to move through a conductor. In scientific terms, it is the specific energy per unit charge, mathematically defined as joules per coulomb. It is similar a to pressure in a fluid tube: the force that moves fluid through a pipe, and is measured in the unit of the Volt (V)."

Connecting Voltmeters

To measure the potential difference across two points, we need to attach the voltmeter in parallel to the two points. If the potential is higher at the + socket, the voltmeter indicates a positive potential difference. If the potential is higher at the – socket, the voltmeter indicates a negative potential difference.

Wrong Connections

Since the Voltmeter has a very large resistance, placing it in series throughout the circuit will cause a very large disturbance throughout the circuit. It would change the current running through the circuit drastically.However if the Voltmeter had low resistance and was placed in parallel, this causes a short circuit to occur.

Ohmmeters

What is an Ohmmeter

An Ohmmeter is a measuring device used to measure the Resistance of a resistor in an electrical circuit. They are created from Ammeters in series with a very small voltage source. When you connect to the unknown resistor, the small voltage pushes a current through the resistor and ammeter. The Ohmmeter is then "calibrated" to display the value of the applied voltage divided by observed current, R= V/I through Ohm's Law. Resistance is also measured in Ohms.

You will also notice that you have to remove the resistor of interest from the circuit before using this device because it is its own device with a voltage source.

Commercial Meters

In reality, these devices are a little more efficient. Digital Voltmeters can read Voltage directly and they are made by testing how long it takes a known current to discharge a capacitor. A Digital Ammeter is a essentially a Voltmeter with a small resistance in parallel; it reads the small potential difference across the resistance. Digital Ohmmeters are created through driving a small known current through an unknown resistor and measuring the potential difference.

Multimeters

Usually all three of these devices can be meshed together to make one final measuring device. This device is often referred to as the Multimeter. It is also known as a VOM device because it can test Voltage, Resistance and Current.

Examples

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Connectedness

1. How is it connected to your major?

I am a Computer Engineering(CompE)(both my major and something that interests me) student here at Georgia Institute of Technology. This is a topic that is extremely important to the world of ECE. We use these measuring devices in numerous real-life applications and building of circuits. They are a vital part to both teaching and understanding how a certain circuit performs.

1. Is there an interesting industrial application?

Nowadays, most of these calculations are done computationally so there is no need for these physical measuring devices. However, some places still use these measuring devices in day to day life.

History

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See also

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