Measuring devices

Chapter Measuring devices

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • 3CHAPTERMeasuring devicesNOW THAT YOU’RE FAMILIAR WITH THE PRIMARY UNITS COMMON IN ELECTRIC-ITYand electronics, let’s look at the instruments that are employed to measure thesequantities.Many measuring devices work because electric and magnetic fields produce forcesproportional to the intensity of the field. By using a tension spring against which theelectric or magnetic force can pull or push, a movable needle can be constructed. Theneedle can then be placed in front of a calibrated scale, allowing a direct reading of thequantity to be measured. These meters work by means of electromagnetic deflectionor electrostatic deflection. Sometimes, electric current is measured by the extent of heat it produces in a re-sistance. Such meters work by thermal heating principles.Some meters work by means of small motors whose speed depends on the mea-sured quantity. The rotation rate, or the number of rotations in a given time, can bemeasured or counted. These are forms of rate meters.Still other kinds of meters actually count electronic pulses, sometimes in thou-sands, millions or billions. These are electronic counters. There are also various othermetering methods.Electromagnetic deflectionEarly experimenters with electricity and magnetism noticed that an electric currentproduces a magnetic field. This discovery was probably an accident, but it was an ac-cident that, given the curiosity of the scientist, was bound to happen. When a mag-netic compass is placed near a wire carrying a direct electric current, the compassdoesn’t point toward magnetic north. The needle is displaced. The extent of the er-ror depends on how close the compass is brought to the wire, and also on how muchcurrent the wire is carrying.Scientific experimenters are like children. They like to play around with things. Most44Copyright © 2002, 1997, 1993 by The McGraw-Hill Companies. Click here for terms of use.