The term solenoid is generally applied to a looping conductor, which has more many loops as required to generate a stable magnetic field. The direction of the magnetic field is straight inside the solenoid, with no component in any other direction than the horizontal. This is when assuming the axis of the solenoid to be the x-axis. The shape of a solenoid is essentially a helix. This helical shape is responsible for the stable magnetic field inside the solenoid as all the non-horizontal components are cancelled out by each other.
The only component of the field that remains is in the direction parallel to the axis. Solenoids make most of the electromagnets that are used in the industry. They are preferred over permanent magnets because the magnetic field can be controlled, and the magnet can be turned on and off at will. This gives the solenoid great flexibility over the permanent magnet.
The term solenoid was first used by Andre Marie Ampere, who is also known as the father of classical electromagnetism. He used the term to refer to any helical coil.
For a solenoid to be properly called a solenoid, the length of the solenoid should be significantly greater than the diameter of the solenoid. If the diameter of the solenoid is not smaller than the length, the field produced by such a solenoid is very unstable and has components in every direction. The coil of the solenoid is not always necessarily straight along its axis. Solenoids come in a variety of shapes, and one such example is the solenoid which was bent into the shape of an electromagnet by English physicist William Sturgeon when he was conducting his experiments with the solenoid around 1824.
In modern times, solenoids are widely used to make transducer devices. Transducers convert energy into linear motion. Thus, the transducer is used to convert electrical energy into mechanical one by using the energy stored in its magnetic field. The field exerts a force on any moving charge as given by the law of Lorentz. This force is used to extract useful work. Solenoids have become a very important part of modern electrical engineering owing to their simple design and effectiveness. They are also widely used in electric motors and generators.
Toroids are essentially solenoids that have been bent to join their two ends. This forms a helical loop. A toroid must have a great number of windings, or it will not operate properly. The shape of a toroid can be understood by considering a doughnut. Just like a doughnut, the toroid has a hole in its centre while the loop runs around that hole. Toroids are usually made from a core of ferromagnetic materials, which means they are soft magnets and can be operated as desired. The main advantage of a toroid over a solenoid is its field containment. The magnetic field of a toroid is completely contained inside the loop. No component of the field is outside.
This magnetic field is directed along the cross-sectional axis of the toroid. This gives an even more stable magnetic field than the solenoid. The core of the solenoid increases its effectiveness by several times. The core makes it a very powerful electromagnet. The core is usually laminated to avoid losses due to eddy currents. Eddy currents arise in a solid conductor due to the electromagnetic forces experienced by the electrons in the conductor. This gives rise to a whirlpool-like current in the conductor's body and causes a significant amount of energy wastage.
In the earlier times, square-shaped cores were used, but engineers quickly turned to toroidal shaped cores because they give higher performance than other closed loops. The main advantage of the toroidal core is its symmetry. The toroidal shape prevents a significant amount of flux leakage outside the toroid. This results in a very low electromagnetic interference from the toroid. The devices in the vicinity are not affected. This also increases the efficiency of the device that employs a toroid. The energy conversion ratio, given by the input energy over the output energy, is maintained at a low value, meaning the device is converting more of the input energy into output energy.
Toroids form an important part of the most efficient electrical devices. They are widely used in DC motors as a stator that helps move the motor's mechanical part. The stable, unidirectional field inside the toroid ensures the high performance of the motor. They are also used in a variety of electronic circuits as power supplies, inverters and amplifiers. Their use as an amplifier finds them in everyday equipment like televisions, computers and radios.