### Re: How do voltmeters work?

Date: Sun Dec 2 04:57:02 2001
Posted By: Nauzad Tantra, Undergraduate, Production/ Industrial engg., D J Sanghvi college of engg.
Area of science: Physics
ID: 1006113403.Ph
Message:
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Hi,
You haven't specified what type of information you need for Voltmeters.
Ill assume you want information about how they work.

Whenever electric current passes through an electric conductor a magnetic
field is set up around it.
(Note: Curl the fingers of your right hand with your thumb pointing
outwards. If the direction of your thumb points in the direction of the
flow of current. Then your fingers will point in the direction of the
magnetic field.) The higher the current the higher is the magnetic field
setup.

This principle is used in many electrical appliances like motors,
generators, solenoids and of course voltmeters and ammeters (Ammeters are
used to measure electric current).

To see this phenomenon hold a magnetic compass close to a motor or
generator, or even an electric wire. Make sure you dont touch the
equipment and have an adult nearby.
Another simple experiment could be done by turning a wire many times
around a iron nail. Connect the ends of the wire to a battery. This will
deflect the magnetic compass. The higher the number of turns the higher
will be the deflection of the compass.

Getting closer to your questions, voltmeters contain a coil of high
resistance suspended between 2 magnets. The ends of the coil are connected
in parallel to the device across which voltage is to be measured. When
current passes through the coil, it deflects since the magnetic field
setup in the coil repulses the magnetic field of the magnets. The coil is
suspended on springs (usually made of phosphor bronze ) which restrict its
movement. The springs deflect to balance the force exerted by the change
in magnetic field in proportion to the current that passes through the
coil.

What follows is some theory on magnetic field effects on current carrying
needed.
Consider the ends of a magnet (say a horseshoe magnet) kept facing each
other at some distance. Put a coil of wire between the ends such that it
can rotate freely about an axis perpendicular to the magnet. If you pass a
current through this coil you will find that the coil will try to turn
(ie: The current will set up a magnetic field opposing that of the magnet.
This will cause a torque to be setup.)
This torque can be calculated by the equation:

T = niAB sinO

Where,
T: Torque
n: Number of turns of the coil
i: Current in the coil
A: Area of each turn
B: Strength of the magnetic field.
O: The angle between the direction of the magnetic field (ie: The
direction of the 2 poles of the magnet from north to south) and a normal
(perpendicular line) to the area of the coil.

For further information you can refer to most physics textbooks. You can
also look up topics like Galvanometers and magnetic effect of electric
current.

References:
1) The Usborne book of Science seems to have good explanations of
electromagnetic effects which you will understand easily.
2)Fundamentals of Physics by Resnick and Halliday seems to have explained
theories in detail, if you really require want to learn all about
electromagnetics.
2) Progressive Physics by Bapat and Mahajan too have covered this topic.

free to email me.

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