# Secondary Instruments

Secondary Instruments are distinguish into two major categories.

#### Secondary Instruments according to Function

According to the function, they are further classify into four sub categories.

• Displaying Instruments: These instruments indicate the reading with the help of waves. These instruments not only show the electrical value but also plot a graph of these values. Example of such instrument is oscilloscope.
• Integrating instruments: Such instruments save the reading in a specific time. Like a KWH meter and Ampere Hours Meter. If they disconnect from the system so they still indicates the reading on the screen.
• Recording Instruments: such instruments, which indicates the reading through a pen on a graph paper, are recording instruments. The mechanical moving system of that instrument is connect with inked pen. By moving 180 degree, it rotates the pen left right.  The tip of the pen touches the graph paper, which plots the reading on it. These instruments are commonly use in Powerhouses and sub stations.
• Indicating Instruments:  indicating instruments are that instruments which indicate the reading by the pointer moving on the screen. These instruments indicates reading unless they are connect to the supply source. When the supply source is disconnect, the pointer goes to the zero position. Example of such instruments are Voltmeter, Ampere meter, Frequency meter etc.

#### According to Mode

According to the mode, they are further classify into two major categories.

Analogue Instruments:  Such instruments, which indicates the value by a pointer on a screen, are analogue instruments. These instruments are widely used in the field.

Analogue Instruments are further Classify into three sub categories.

• Electromechanical Instruments: Such instrument the technique use for the rotation of pointer is due to electromagnetic induction.
• Electronic Instruments: This instrument can detects a less value of signal and amplify it. These amplify signals can cause the deflection of a pointer as a result it show the reading on the screen.
• Electromechanical and electronic signal:  Such an instrument have both characteristics of electromechanical and electronic instruments.

Digital Instruments: Such instruments indicates the reading of the measured value and display it in the digital screen. These Screens are usually of LED (Light Emitting Diode) of LCD (liquid crystal display) type. Digital meters are commonly use worldwide. These instruments did not have the pointers, while they have logic gates instead due to which the current, voltage, ampere, frequency and some other electrical quantities are measured.