When an electrical charge is moving or an electric current passes through a wire, a circular magnetic field is created.
This can be seen with iron shavings or filings on a card that will align in the magnetic field when a current is passed through a nearby wire.
This field can be demonstrated by placing fine iron filings or shavings on a car and sticking a wire through the middle of the card. Certain conventions are used to determine the direction of the magnetic field with respect to the direction of the current. The convention for a magnetic field is that the lines of force move from north (N) to south (S). The right-hand rule for determining the direction of the magnetic field is that if you wrapped your right hand around a wire with your thumb pointing in the direction of the electrical current, then your fingers would be in the direction of the magnetic field.
You can verify the direction of the magnetic field by placing one or more compasses on a card and observing their direction. Please include it as a link on your website or as a reference in your report, document, or thesis. Control devices are needed to start, stop, or redirect current flow in an electrical circuit.
A single-pole input, double-throw output switch has one wire going it and two wires coming out.
The momentary contact switch has a spring-loaded contact that keeps it from making the circuit except when pressure is applied to the button. A variation of this type is the normally closed (not shown) which works the opposite as described above. The flasher operates basically the same as the time delay switch; except when the contacts open, current stops flowing through the heating element.
A relay is simply a remote-control switch, which uses a small amount of current to control a large amount of current. A solenoid is an electromagnetic switch that converts current flow into mechanical movement. Circuit protection devices are used to protect wires and connectors from being damaged by excess current flow caused by either an over current or short-circuit. Fuses, fuse elements, fusible links, and circuit breakers are used as circuit protection devices. The blade type fuse is a compact design with a metal element and transparent insulating housing which is color-coded for each current rating. Many older vehicles, both foreign and domestic, use glass or ceramic fuse cartridges that were either color coded or stamped on case for current ratings.
Fusible links are divided into two categories: the fuse element cartridge and the fusible link. Fusible links are short pieces of a smaller diameter wire designed to melt during an over current condition. Circuit breakers are used in place of fuses for the protection of complicated power circuits such as the power windows, sunroofs and heater circuits. A circuit breaker basically consists of a bimetal strip connected to two terminals and to a contact in between. The circuit breaker contains a metal strip made of two different metals bonded together called a bimetal strip.
Using standard conventions for the direction of electric current and magnetic lines of force, the direction of the magnetic can be determined by what is called the right-hand rule for magnetic field.


Since electrical current moving through a wire consists of electrons in motion, there is a magnetic field around the wire. When a DC electrical current is passed through the wire, the iron filings align to show the magnetic field. It is an unfortunate choice that was made years ago, but you need to be aware of the convention. We don't know if they move in that direction or not, but it is just a convention that everyone follows.
Otherwise with AC, the direction of the current and magnetic field will alternate 50 or 60 times a second. Iron filings on a card can demonstrate the magnetic field when current is passed through a nearby wire.
Most switches require physical movement for operation while relays and solenoids are operated with electromagnetism.
In an engine coolant temperature switch, when the coolant reaches the temperature limit, the bimetal element bends causing the contacts in the switch to close. Circuit protection devices are available in a variety of types, shapes, and specific current ratings.
Common locations include the engine compartment, behind the left or right kick panels, or under the dash. Three different types of blade fuses exist; The Maxi Fuse, The Standard Auto fuse, and the Mini fuse.
Glass fuses were used on older domestic vehicles while the ceramic were used on most older European vehicles. The construction and function of fusible links and fuse elements are similar to that of a fuse. A colored plastic housing contains the fusing portion element which can be viewed through a clear top. A fusible link is usually four (4) wire sizes smaller than the circuit that it is protecting. Three types of circuit breakers exists: The manual reset type - mechanical, the automatic resetting type - mechanical, and the automatically reset solid state type - PTC. Manual circuit breaker when tripped (current flow beyond its rating) will open and must be reset manually. Using standard conventions for the direction of electric current and magnetic lines of force, the direction of the magnetic can be determined by what is called the right-hand rule.
Electronic control devices include capacitors, diodes, and transistors, but those will be discussed in another training module.
Switches are referred to as SPST (single-pole, single-throw), SPDT (single-pole, double-throw), or MPMT (multiple-pole, multiple-throw).
As the switch is rotated (moved from true vertical) the mercury flows to the opposite end of the capsule with the contacts, completing the circuit. As current flows through the switch, current flows through the heating element causing it to heat, which causes the bimetal strip to bend and open the contacts. The bimetal strip returns to the rest position which closes the contacts, allowing current to flow through the contacts and heating element again.
Relay construction contains an iron core, electromagnetic coil, and an armature (moveable contact set).
The magnetic field attracts the upper contact arm and pulls it down, closing the contacts and allowing power from the power source to go to the load.


If an iron core is placed near the coil that has current flowing through it, the magnetic field will stretch out like a rubber band, reaching out and pulling the iron bar into the center of the coil.
The element in the fuse melts, opening the circuit and preventing other components of the circuit from being damaged by the overcurrent. Fuses are usually grouped together and are often mixed in with other components like relays, circuit breakers, and fuse elements.
Ceramic fuses used an amperage color rating system while glass fuses have the amperage ratings stamped into one of the metal end caps.
The main difference is that the fusible link and fuse element are used to protect higher amperage electrical circuits, generally circuits 30 amps or more.
When heat from the excessive current is higher than the circuit breaker current rating, the two metals change shape unevenly. Each switch sends current from different source to different output circuits at the same time depending on position. Mercury switches are often be used to detect motion, such as the one used in the engine compartment on the light. As current continues to flows through the heating element, the bimetal strip is kept hot, keeping the switch contacts open. There are two types of relays: normally open (shown below) and normally closed (NOT shown). As with fuses, once a fusible link or fuse element blows out, it must be replaced with a new one. The dotted line between the switches indicates they move together; one will not move without the other moving as well. The amount of time delay before the contacts open is determined by the characteristics of the bimetal strip and the amount of heat produced by the heating element.
Common uses for this type of switch are the turn signals or the four-way flasher (hazard lamps).
When the control circuit switch is closed, however, current flows to the relay and energizes the coil. Remember, excessive current causes excess heat, and it's the heat and not the current that causes the circuit protector to open. Although fuse elements are available in two physical sizes and are either plug in or bolt on design, the plug-in type is the most popular. When power to the switch is turned off, the heating element cools and the bimetal strip returns to the rest position and the contacts are closed. The resulting magnetic field pulls the armature down, closing the contacts and allowing power to the load. Many relays are used for controlling high current in one circuit with low current in another circuit.
An example would be a computer, which controls a relay, and the relay controls a higher current circuit.



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