E1C06 09/14/2010
11:55:2 Page 211
Most devices that use the D’Arsonval movement employ a pointer whose deflection increases
with the magnitude of current applied. This mode of operation is called the deflection mode. A
typical circuit for an analog current measuring device, an ammeter, is shown in Figure 6.4. Here the
deflection of the pointer indicates the magnitude of the current flow. The range of current that can be
measured is determined by selection of the combination of shunt resistor and the internal resistance
of the meter movement. The shunt resistor provides a bypass for current flow, reducing the current
that flows through the movement. A make-before-break switch prevents current overload through
the meter coil.
A galvanometer is a measuring device used to detect a current flow in a circuit. It is a highly
sensitive D’Arsonval movement calibrated about zero current; the indicator normally indicates zero
but can deflect to the plus or minus direction. The pointer is used to adjust a circuit to a zero current
state. This mode of operation is called the null mode. The highest sensitivity of commercial
galvanometers is approximately 0.1 mA/division for a galvanometer with a mechanical pointer.
That’s quite sensitive.
Pointer
Full
scale
Coil assembly
Permanent
magnet
Restoring
spring
Jeweled
pivot
bearings
N
S
Figure 6.3 Basic D’Arsonval meter
movement.
Current vectors
( , in: , out)
T
F
F
n
Figure 6.2 Forces and resulting torque on a
current loop in a magnetic field.
6.2 Analog Devices: Current Measurements 211
11:55:2 Page 211
Most devices that use the D’Arsonval movement employ a pointer whose deflection increases
with the magnitude of current applied. This mode of operation is called the deflection mode. A
typical circuit for an analog current measuring device, an ammeter, is shown in Figure 6.4. Here the
deflection of the pointer indicates the magnitude of the current flow. The range of current that can be
measured is determined by selection of the combination of shunt resistor and the internal resistance
of the meter movement. The shunt resistor provides a bypass for current flow, reducing the current
that flows through the movement. A make-before-break switch prevents current overload through
the meter coil.
A galvanometer is a measuring device used to detect a current flow in a circuit. It is a highly
sensitive D’Arsonval movement calibrated about zero current; the indicator normally indicates zero
but can deflect to the plus or minus direction. The pointer is used to adjust a circuit to a zero current
state. This mode of operation is called the null mode. The highest sensitivity of commercial
galvanometers is approximately 0.1 mA/division for a galvanometer with a mechanical pointer.
That’s quite sensitive.
Pointer
Full
scale
Coil assembly
Permanent
magnet
Restoring
spring
Jeweled
pivot
bearings
N
S
Figure 6.3 Basic D’Arsonval meter
movement.
Current vectors
( , in: , out)
T
F
F
n
Figure 6.2 Forces and resulting torque on a
current loop in a magnetic field.
6.2 Analog Devices: Current Measurements 211