MadSci Network: Physics

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

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 

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 

What follows is some theory on magnetic field effects on current carrying 
conductors. This will help you do some calculations for your project if 
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

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 

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 
2) Progressive Physics by Bapat and Mahajan too have covered this topic.

Hope the above information helps. If you need more information please feel 
free to email me.

Nauzad Tantra

Current Queue | Current Queue for Physics | Physics archives

Try the links in the MadSci Library for more information on Physics.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2001. All rights reserved.