is one of the schematic symbols used for this solenoid. Figure 3-1, view C, shows a simple relay. One of
the schematic symbols for this relay is shown next to the relay.
Q1. What are three reasons circuit control is needed?
Q2. What are the three types of circuit control devices?
Q3. Label the schematic symbols shown in figure 3-2.
Figure 3-2.Schematic symbol recognition.
There are thousands and thousands of switch applications found in home, industry, and the Navy.
Hundreds of electrical switches work for you everyday to perform functions you take for granted. Some
switches operate by the touch of a finger and many others are operated automatically.
Switches are used in the home to turn off the alarm clock, to control the stove, to turn on the
refrigerator light, to turn on and control radios and televisions, hair dryers, dishwashers, garbage
disposals, washers and dryers, as well as to control heating and air conditioning. A typical luxury
automobile with power seats and windows might have as many as 45 switches.
Industry uses switches in a wide variety of ways. They are found in the business office on computers,
copy machines, electric typewriters, and other equipment. A factory or shop may use thousands of
switches and they are found on almost every piece of machinery. Switches are used on woodworking
machinery, metal working machinery, conveyors, automation devices, elevators, hoists, and lift trucks.
The Navy uses switches in a number of ways. A typical aircraft could have over 250 switches to
control lights, electronic systems, and to indicate whether the landing gear is up or down. Ships, fire
control systems, and missile launchers are also controlled by electrical switches. In fact, almost all
electrical or electronic devices will have at least one switch.
Switches are designed to work in many different environments from extreme high pressure, as in a
submarine, to extreme low pressure, as in a spacecraft. Other environmental conditions to consider are
high or low temperature, rapid temperature changes, humidity, liquid splashing or immersion, ice,
corrosion, sand or dust, fungus, shock or vibration, and an explosive atmosphere.