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Theory:

Switch:
An electric switch or a key is connected to the circuit to stop or allow the current flow.
switch.png
A toggle switch
 
The symbol for the switch is shown below:

switch-w422.png
Symbol of a switch
If a key is switched off (open), the current will not flow across the circuit; thus, it is an open circuit. A lamp does not glow in this type.
If the key is switched on (closed), the current will flow, making it a closed circuit. A lamp glows in this type.
shutterstock_1707749467.jpg
Open and closed circuits
 
Ammeter:
An ammeter is an instrument used for measuring electric current in a circuit. The electric current is measured in the units of ampere.
 
136910945444f0b389176b.jpg
An ammeter
 
Ammeter.png
Symbol of ammeter
 
Voltmeter:
A voltmeter is an instrument used for measuring the potential difference in a circuit. The potential difference is measured in the units of volts.
 
Voltmeter_in_classroom.jpg
A voltmeter
 
Voltmeter_symbol.png
Symbol of voltmeter
  
Galvanometer:
A galvanometer is an instrument used to find the direction of the current.
  
Galvanometer_-_001.jpg
A galvanometer
  
galvanometer.png
Symbol of galvanometer
  
Resistor:
The resistor is used to control the amount of current flowing through the circuit. The resistor can be fixed or variable.
 
Fixed resistor:
A fixed resistor has a definite or a fixed value that cannot change.
 
rsamurti-RSA-IEC-Resistor-Symbol.png
Symbol of a fixed resistor
   
Variable resistor:
A variable resistor or a rheostat controls and selects the amount of current flowing through a circuit.
 
rsamurti-RSA-IEC-Variable-Resistor-Symbol-1.png
Rheostat
 
The components such as a diode, Light Emitting Diode (LED), seven-segment display and various other connections will be discussed in detail in upcoming sections.
Reference:
https://upload.wikimedia.org/wikipedia/commons/d/da/Ammeter.png
https://upload.wikimedia.org/wikipedia/commons/5/59/Voltmeter_symbol.png
https://upload.wikimedia.org/wikipedia/commons/6/67/Voltmeter_in_classroom.JPG
https://upload.wikimedia.org/wikipedia/commons/8/8a/Galvanometer_-_001.jpg
https://www.flaticon.com/free-icon/galvanometer_114771
https://freesvg.org/img/rsamurti-RSA-IEC-Resistor-Symbol.png
https://freesvg.org/img/rsamurti-RSA-IEC-Variable-Resistor-Symbol-1.png