Chapter Ammeters

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

  • AmmetersGetting back to electromagnetic deflection, and the workings of the galvanometer, youmight have thought by now that a magnetic compass doesn’t make a very convenienttype of meter. It has to be lying flat, and the coil has to be aligned with the compass nee-dle when there is no current. But of course, electrical and electronic devices aren’t allturned in just the right way, so as to be aligned with the north geomagnetic pole. Thatwould not only be a great bother, but it would be ridiculous. Imagine a bunch of scien-tists running around, turning radios and other apparatus so the meters are all lying flatand are all lined up with the earth’s magnetic field! In the early days of electricity andelectronics, when the phenomena were confined to scientific labs, this was indeedpretty much how things were.Then someone thought that the magnetic field could be provided by a permanentmagnet right inside the meter, instead of by the earth. This would supply a strongermagnetic force, and would therefore make it possible to detect much weaker currents.It would let the meter be turned in any direction and the operation would not be af-fected. The coil could be attached right to the meter pointer, and suspended by meansof a spring in the field of the magnet. This kind of meter, called a D’Arsonval move-ment, is still extensively used today. The assembly is shown in Fig. 3-4. This is the ba-sic principle of the ammeter.48 Measuring devices3-4The D’Arsonval meter movement. The spring bearing is not shown.A variation of this is the attachment of the meter needle to a permanent magnet,and the winding of the coil in a fixed form around the magnet. Current in the coil pro-duces a magnetic field, and this in turn generates a force if the coil and magnet arealigned correctly with respect to each other. This meter movement is also sometimescalled a D’Arsonval movement. This method will work, but the inertial mass of the per-manent magnet causes a slower needle response. This kind of meter is also more proneto overshoot than the true D’Arsonval movement; the inertia of the magnet’s mass, once