|MadSci Network: Engineering|
Multimeters are devices which are used to measure various parameters of electric current which include, resistance, voltage and current. (Modern electronic multimeters can also measure other parameters such as frequency, capacity (of capacitors), breakdown voltage (for diodes), and check devices like transistors. Since you have not specified whether you want to know about digital multimeters or the "traditional" analogue one's Ill restrict the discussion to the analog multimeters (The ones with the moving arm or wiper) since they are more common. 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. 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. You'll see the compass needle deflect from its position as you near the wire. 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. Keep the nail on top of a compass. This will deflect the compass needle. The higher the number of turns on the nail, the higher will be the deflection of the compass. Getting more specific, voltmeters contain a coil having high resistance suspended between 2 magnets. The top of the coil is connected to the arm or needle of the multimeter. 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 away from its mean position 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 control its movement. The springs deflect proportional to the force exerted on them (due to the changing magnetic field). By varying the resistance of the coil, you can vary the sensitivity (and hence the range) of the equipment. An ammeter is very similar to the voltmeter. The only differance is that an ammeter has very little resistance to the flow of electric current, and it is connected in series with the source of current. Digital multimeters work on very different principles and have no coils or moving parts. They also usually have a higher range and better accuracy. 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. Hope the above information is sufficient. If you need more information please feel free to contact me. Nauzad Tantra
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