Mohamed Salah & Emad Hamam

Voltmeter

Galvanometer

A galvanometer is a type of ammeter:

produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field.

Sensitive galvanometers were used to detect signals from long submarine cables, and were used to discover the electrical activity of the heart and brain

.

What is a Voltmeter ?A voltmeter is an instrument used for

measuring electrical potential difference between two points in an electric circuit.

voltmeters were originally called galvanometers

Technically, all voltmeters are ammeters, as they measure current rather than voltage.

1- Analog:

Types Of Voltmeters:

2-Digital

Digital meters can be made with high accuracy, typically better than 1%.

Voltmeter Design

An ideal voltmeter has infinite resistance: It is an open circuit. Although it is impossible to make a physical voltmeter with infinite resistance, a well designed voltmeter exhibits a very large internal input resistance.

A voltmeter is placed in parallel with a circuit element to measure the voltage drop across it and must be designed to draw very little current from the circuit so that it does not appreciably change the circuit it is measuring

Determine the internal resistance of the voltmeter in the circuit shown in figure :

Example

The voltmeter reads the voltage across itself, which includes its internal resistance.

The current is given by the equation:

From Ohm's Law, if we know the current (I) and the voltage we can compute .

V=R I V=(Rs+Rg)Ig V=Rs Ig + Rg Ig Rs=Vfull range/Ig - Rg

How to find (Rs)?

Multi Range Voltmeter

 If you have electric current equal to 1 (mA) and

you have a light bulb resistance of 200 ohms. How much is the needed voltage to turn the bulb on?

calculate the voltage through Ohm's law:

V = I * RV= 200 * 0.001V = 0.2 voltsSo you need a battery voltage difference of 0.2 voltage.

 

(ُexample:

Measure ground resistance using :- lamp

voltmeter

Ammeter

Note: This method gives approximate results .

Applications:

connect the circuit as before .

take the voltmeter and ammeter values .

Voltage along the ground resistance :-= voltage source - lamp voltage .

Ground resistance = Voltage along the ground resistance / current

How to solve this problem???!!!

 In an experiment to measure the resistance of the

ground by ordinary lamp, voltmeter ,Ammeter . The voltmeter reading which was connected in parallel with the lamp was equal to (217) volts and the reading of the Ammeter respectively connected with the department equal to (0.45) mA

.Calculate the resistance of the ground. note that the electric voltage source equals (220 volts)..??

Example:-

Voltage along the ground resistance = voltage source - lamp voltage = 220-217 = 3 V.

Ground resistance = Voltage along the ground resistance / current. 3(v) /.45 (A) = 6.67 ohm .

(Solution) :-