Electromagnets and their Uses

It is easily seen that in solenoid there is a complete field around each turn. These fields are somewhat modified by the adjacent turns, there being many straight lines coming out from the sides of the coil. Should the coil be wound on a core of soft iron of high permeability, the core would absorb these straight lines and give a concentrated field from one end of the core to the other. We then have an electromagnet. We use soft iron not only because of its having high permeability, but because its low retentivity allows very little residual magnetism when the current is turned off. The strength of the magnet increases with the number of amperes flowing and also with the number of turns. The product of amperes and turns is called ampere turns. The strength of an electromagnet with a given core is known to be proportional to the number of ampere turns, the strength of the field depending on the shape of the core. If the poles were brought together into a U shape, the field would become stronger. Of course, the two legs of the U must have opposite poles.

The most obvious use of the electromagnet is in lifting iron weights. They are often capable of holding pieces of iron or steel weighing thousands of pounds; they do not slip, as do hooks and ropes; and they can be operated by the throw of a switch at a distance. We know small, powerful electromagnets to be used by doctors to remove steel particles from the eye. Among the common applications of the electromagnet are the electrical bell, the telephone, the telegraph, radio loud-speaker, circuit breakers, relay for remote control of machines, electrical measuring instruments, motors, and generators.

Semiconductors

A review of the mechanism for conducting electricity through various kinds of matter shows that in electrolytes and in gases conduction occurs through the motion of ions, that in metals conduction takes place through the motion of electrons, and that in insulators there is no conduction but only a slight displacement of the charges within the atoms themselves. There is still another kind of matter in which conduction takes place by electrons just as in metals, but, contrary to the behaviour of metals, a substance of this kind exhibits an increase of resistance as the temperature falls. Such a substance is referred to as a semiconductor, and at the absolute zero of temperature it would be an insulator. Among the examples of semiconductors the most important at present seem to be silicon and germanium.

The variation of resistance with temperature is accounted for as follows. In metal only a very few electrons are free to move upon application of a potential difference. The temperature of the metal being lowered, the thermal vibration of its atoms is reduced; as a result the atoms interfere less with the motion of the electrons, and consequently the resistance is lowered. Those electrons free to move in a metal are in semiconductors bound loosely to the atoms. At absolute zero a semiconductor has no current carriers. The temperature being raised, more and more of the loosely bound electrons are released by the thermal energy and conduction is improved, which means that the resistance is lowered as the temperature rises.