If we allow a current to flow through a coil of wire, it will generate a magnetic field. That magnetic field can be used to move nearby permanent magnets or ferrite metal components. We say that there is an induced magnetic field radiating from the coil of wire. When the induced magnetic field cuts, or passes through, the magnetic field of the permanent magnet, it has the same effect of two magnets cutting each other's fields. In other words, it attracts or repels according to polarity.
We have seen how this can be used to our advantage in the case of the
relay, but it has much more potential than that. If, for instance, we
drill a hole in a magnet, and put an axle through it. If we mount the
axle on a stand, we can spin the magnet upon its axle by hand.
Now if we place a coil near the magnet, we can make the magnet turn by
controlling the polarity of the current through the wire.
If we polarize the coil, such that the north side of the electromagnet
is facing the permanent magnet, it will cause the north pole of the
magnet to rotate away from the coil, while attracting the south side of
the magnet toward the coil. The magnet spins 180 degrees.
- If we then change the polarity of the battery, so that the south
side of the coil faces the permanent magnet, it causes the magnet to
turn another 180 degrees, for a total of 360 degrees. We have caused
the magnet to spin 360 degrees, and in effect, created a crude form of
Permanent Magnet Coil Hair Spring Pointer ScaleIn the D'Arsonval movement, the permanent magnet is fixed. It is the coil which does the turning. The coil is mounted on a needle fine axle, which would allow the coil to spin 360 degrees. The hair spring is used to return the needle to its original position, as well as to regulate the movement of the meter. The pointer, which is attached to the turning coil, is used for an indicator of how far the coil has turned. Finally, the scale is used as a numerical standard to compare readings.
The D'Arsonval movement can be used by itself as a standalone instrument called a GALVANOMETER. The galvanometer is a device which indicates the presents of electrical current. It is not calibrated for Ohms, Volts, or Amps.
By adding a high resistance in series with the D'Arsonval movement, we create a VOLTMETER.
A Voltmeter is a device used to measure electrical potential in Volts. The series resistor is called a MULTIPLIER, and its purpose is to limit the flow of current through the fragile meter movement. Given a known resistance, the Voltage read at the leads of a Voltmeter can be exactly calculated to cause a certain amount of current to flow through the coil of the meter. Armed with Ohms Law, and knowing the value of the resistor we use, we can calibrate the meter's scale to measure an exact amount of Voltage. We know that:
E Where: R = Multiplier Resistance R = --- E = Full Scale Voltage I I = Full Scale Reading of MeterSo it follows that given a meter movement that deflects full scale when 1 milliampere flows through it, We can find the value of the multiplier resistor that is necessary by using the following formula:
1000 x E R = --------- IIf we measure the Voltage across the circuit in the diagram above, we find that E = 400 Volts.
(NOTE THAT VOLTAGE IS ALWAYS MEASURED IN PARALLEL),
1000 x 400 400K R = ----------- = --------- I 1.0 AmpsKnowing, then, that we have a 400KW Resistor, and it requires a 400 Volt potential to cause full deflection we divide the meter resistance by the full scale voltage and come up with the sensitivity of the meter.
meter resistance 400K Ohms R = ----------- = --------- = 1000 Ohms per Volt sensitivity. full scale 400 Volts voltage
Other Types of Meters
Or what if you wanted to know the amount of current flowing through a circuit, so that you knew what size of fuse to put in the circuit? You would need an Ammeter to measure the current in Amps.
Recall that the Voltmeter had a resistor in series with it. This resistor, called the "Multiplier Resistor" was used to calibrate the meter to work within a given range. A Voltmeter is also placed in parallel with the circuit in test.
An Ammeter, on the other hand, is built with a resistor in PARALLEL or in SHUNT with the D'Arsonval movement's coil. In the case of the Ammeter, the SHUNT resistor is of a very low resistance. Much lower resistance, in fact, than the coil in the meter movement. Remember finding resistance in a parallel circuit? The two resistors in parallel carry more current than either of the resistors by themselves. This is because the combined resistance is lower than the lowest resistor in the parallel network. Also, the resistor with the lowest resistance always carries the greatest current. This is of utmost importance here. If too much current were to go through our sensitive meter coil, it would burn up and destroy the coil, hence making the meter useless. The answer, of course, is to make sure that no matter HOW high the current, the majority of the current will always flow through the shunt resistor. It is for this reason that the shunt resistor has a lower resistance value than the coil winding of the meter itself. The formula we use for finding the value of the shunt resistor is as follows:
= shunt resistance
R m = meter movement resistance
I m = full scale meter movement current
I s = shunt current
Finally, the Ohmmeter is one of the most used tools on the electronics workbench. It is used not only to measure the resistance value of a given resistor or circuit component, but also to check continuity of wire, to test for opens and shorts in a circuit, and many other things. But an ohmmeter is not self sufficient.
The Ohmmeter is made up of an Ammeter, a battery, and a
CURRENT LIMITING RESISTOR
. As shown in the picture to the side, the battery causes a current to flow
through the meter. We know the value of the current limiting resistor, so if
we short the meter leads together, we know how much current the meter should
indicate. If the meter indicates a lower current value than we expected, then
there must be some added resistance. Therefore, we can use this device to
detect, and to measure the value, of an outside resistance.