Secondary Instrument

The secondary instruments are those which are so designed that the value of current, voltage or any other quantity to be measured can be determined from its deflection only if the instrument has been compared with an absolute. In other words, secondary instruments need calibration.

The secondary instrument is usually used at large absolute instruments are only used in standard laboratories. examples are PMMC, Moving iron instruments etc.

1. Indicating Instrument:

The indicating instruments indicate the R.M.S. value of the electrical quantity. which exists at the moment when the measurement is being done. The indication is by means of the deflection of the pointer on a calibrated scale. for example, Ammeter, Voltmeter, Wattmeter etc. as shown in the following diagram.

You can observe this in the above diagram. we have two poles i.e North and South and other components are required for Indicating instruments such as Moving coil, scale with pointer, deflecting torque etc. Current flow from moving coil in indicating instrument.

2. Recording instrument:

The Recording instruments are those which not only indicate the existing value of an electrical quantity being measured. but also provides a record of variations in it over a predetermined period of time. for example Cardiogram, chromatogram, X-Y plotter etc. as shown in the figure.

3. Integrating Instrument:

The integrating instrument measures, registers, and integrates the electrical quantity being measured over a pre-defined period. The integrated added value is displayed with the help of a set of scales and pointers. for example odometer, energy meter etc. as shown in fig below.

Principles of operation of Electrical Instruments:

Electrical measuring instruments work on the principle of operation of different effects such as Magnetic effect, Electro-dynamic effect, Electromagnetic Induction, thermal Effect, Chemical Effect, Electrostatics Effect, etc. The principle of operation of the different electrical instruments is given in the table below-

Type	Effect	Suitable for	Work as
Moving Iron	Magnetic Effect	DC, AC	Ammeter, Voltmeter
PMMC	Electro-dynamic Effect	DC	Ammeter, Voltmeter
EMMC (Dynamometer)	Electro-dynamic Effect	DC, AC	Ammeter, Voltmeter, Wattmeter
Induction Type	Electro-Magnetic Induction	AC	Ammeter, Voltmeter, Wattmeter, Energy Meter
Hot Wire	Thermal Effect	DC, AC	Ammeter, Voltmeter
Electrolyte Meter	Chemical Effect	DC	Amp-Hr Meter
Electrostatics Meter	Electrostatics Effect	DC, AC	Voltmeter

Note: Kindly rotate your screen for full table visualization.

Difference between Analog and Secondary instruments:

Characteristics	Analog	Secondary
Definition	Those instruments which give the value of the quantity to be measured in terms of the constant of the instrument and its deflection are called absolute instruments.	Those instruments in which the electrical quantity being measured is given directly by the deflection of the instrument are called secondary instruments.
Environmental conditions	Used in any type of environmental conditions.	Digital instrument involves electronic devices which required proper environmental conditions.
Effect of Electronic noise	More susceptible to electronic noise.	Less affected by electronic noise.
Accuracy	Less accurate	More accurate
Moving parts	Present	Not present
Cost	Less	More than analog instrument
Sensitivity	More sensitivity	Less sensitivity
Size	The size of analog instruments is larger than a Secondary instrument.	Digital instruments are comparatively smaller than analog instruments.
Extra power supply required	Not required an Extra power supply	Required extra power supply
Example	PMMC, EMMC, MI etc	digital multimeter, digital ammeter and voltmeter, clamp-meter, etc.