This document discusses different types of meters used to measure electrical values - ammeters, voltmeters, ohmmeters, and multimeters. It provides details on:
- How ammeters are based on galvanometers and have a shunt resistor added in parallel to measure higher currents.
- Examples of converting a galvanometer to an ammeter and voltmeter by adding appropriate resistances in parallel or series.
- How ohmmeters use a galvanometer and reference resistor to measure unknown resistances.
- The Wheatstone bridge circuit for measuring unknown resistances using a balanced bridge and known resistors.
4. Most of the time these three
meters are combined to form
a MULTIMETER that can be
switched from one to another.
There are 2 types of such
meters: those who use digital
electronics and those that do
not.
5.
A galvanometer is a type of
sensitive ammeter: an instrument for
detecting electric current. It is
an analog electromechanical transducer that
produces a rotary deflection of some type of
pointer in response to electric
current flowing through its coil in
a magnetic field. It was named after the
italian anatomist Luigi Galvani who became
famous during his time because of his
experiments on “animal electricity”using
frogs as specimen.
6.
The ammeter is a galvanometer with
a low resistance, called shunt
resistor, connected parallel to it.
The shunt resistor provides a bypass
for the current in excess of the
galvanometer’s full scale limit.
7. A galvanometer has a coil resistance of 50.0Ω and
fullscale current of 10mA. Convert this to an ammeter
reading 1.0 A full scale.
GIVEN:
resistance= 50 Ω
Current= 1.0 mA
Scale= 10 A
RECQUIRED:
Convert this to an ammeter
SOLUTION:
To convert the galvanometer to an ammeter, we have we have
to connect a resistor Rp parallel to it. Let Ip and Vp be the
current passing through the voltage across resistor Rp,
respectively.
8. SOLUTION:
To convert the galvanometer to an ammeter, we
have we have to connect a resistor Rp parallel
to it. Let Ip and Vp be the current passing
through the voltage across resistor Rp,
respectively. Let RG , IG and VG represent the
resistance, the current passing through and the
voltage across the galvanometer, respectively.
We are given that RG =50.0 Ω and
IG =1.0mA= 0.001 A. We are also given that the
total current I= 10.0A.
by law of parallel scissors,
9.
by law of parallel scissors,
I= IG + IP
10 A = 0.001 A+ IP
solving for IP,
IP = 9.999A
again by law of parallel scissors,
V T = VP = V G
using ohm’s law,
IP RP = IG RG
substituting values,
(9.999A) RP = ( 0.001 A) (50 Ω)
RP = 0.005 Ω
10. Convert the galvanometer in the preceeding
example to a voltmeter reading 3.0 V full scale.
GIVEN:
Resistance= 50 Ω
Current= 0.001 A
RECQUIRED:
Convert to galvanometer
11.
SOLUTION:
Using ohm’s law we determine the maximum
voltage it can measure.
V=IR
V=(0.001 A)(50 Ω)
V=0.05V
To convert this to a voltmeter that can read up to 3.0
V, a resistance Rs must be connected in series with
the galvanometer,
IG = IS = 0.001 A
Vtotal = VG + VS
Vs = 2.95 V
12.
But VS = IS RS
2.95 V= (0.001 A) RS
solving for RS,
Rs =2950 Ω
13.
Resistance may be measured directly by an
ohmmeter. A simple ohmmeter consists of a
galvanometer in series with a battery and a
resistor R1 , as shown in the fig 11-10. The value
of R1 is chosen so that when the 2 terminals x
and y are made to touch each other, the
galvanometer gives full-scale deflection. A full
scale deflection of the galvanometer indicates
zero resistance between x and y. when x and y
are connected across an unknown resistance R,
the galvanometer will deflect depending on the
value of R. hence the scale can be calibrated to
measure the resistance.
14.
The voltmeter-ammeter method of determining
resistance requires that the current passing through the
resistor as well as the potential difference across the
resistor be measured by an ammeter and a voltmeter,
respectively. The resistance is computed using ohm’s
law.
The Wheatstone bridge is an electrical circuit used for
comparing resistances. It was invented by Hunter
Christie but named after sir Charles Wheatstone who
popularized the use of the electrical circuit. It consists
of a battery, galvanometer and four resistors. The 4
resistors are divided into 2 parallel branches, each
brand consisting of 2 series resistors. Of the four
resistors, 3 are of known while one is unknown. To
determine the value of the unknown resistor , the
known resistances are adjusted and balanced until the
current passing through the galvanometer is zero. The
unknown resistance may be found using the following
relation.
15.
An unknown resistance RS is being measured
by means of a Wheatstone bridge. The
galvanometer reads zero when the other
resistors R1, R2 and R3 are 3 Ω, 2 Ω and 10 Ω,
respectively. Find the unkown resistance Rx .
Refer to the figure 11-11.
