Page 68 - Teach Yourself Electricity and Electronics

48 Measuring devices

Ammeters

Getting back to electromagnetic deflection, and the workings of the galvanometer, you
might have thought by now that a magnetic compass doesn’t make a very convenient
type 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 all
turned in just the right way, so as to be aligned with the north geomagnetic pole. That
would 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 flat
and are all lined up with the earth’s magnetic field! In the early days of electricity and
electronics, when the phenomena were confined to scientific labs, this was indeed
pretty much how things were.
Then someone thought that the magnetic field could be provided by a permanent
magnet right inside the meter, instead of by the earth. This would supply a stronger
magnetic 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 means
of 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.

3-4 The 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 are
aligned correctly with respect to each other. This meter movement is also sometimes
called 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 prone
to overshoot than the true D’Arsonval movement; the inertia of the magnet’s mass, once