Electromagnets always find an application when it is de­sirable to convert electrical power into mechanical power.

Telegraph systems and telephones, relays, motors and generators, radio sets and television sets, electrical measuring instruments as well as thousands of other valuable and neces­sary devices are known to contain electromagnets. They may be used, as well, to protect electrical circuits against over­loads and underloads.

One of the first applications of the electromagnet was in telegraphy. Shilling, corresponding member of the Peters­burg Academy of Sciences was the first to construct the electromagnetic telegraph. He demonstrated his invention as far back as 1832.

As mentioned above, the telephone also uses electromag­nets, uses many of them, in fact. As soon as man learned to send word over the long wires of the telegraph circuit, the next problem to be solved was the telephone. Was it not pos­sible to send the spoken word over similar wires? As a matter of fact, the first practical telephone was invented by the Amer­ican scientist Bell in 1876 and was further improved by Edison.


A fuse is a safety device that protects electric circuits from overloads. It must be put in every circuit where there is danger of overloading the line. All the current used must pass through the fuse, the latter being the weakest link in the circuit.

Such a safeguard is quite necessary. For no matter how careful you are, there is always the possible danger of a short-circuit, or an overload on some appliance. This means that an unusually large electric current will flow. If a short circuit or overload caused more current to flow than the car­rying capacity of the wire, the wire would become hot. It might set fire to the insulation were it not for the fuse.

What actually takes place is simple enough. The fuse wire has a lower carrying capacity than the circuit it protects. Its melting temperature is low. Thus, the fuse will get hot and melt if the current becomes too great. In other words, if the current flow is greater than the carrying capacity of the fuse, the latter melts resulting in an open circuit.

As said before, all conductors have some resistance. In any conductor, heat is caused by the current's flow overcoming this resistance. The greater the current, the hotter the wire becomes. That is why the fuse melts when there is an overload The heat produced automatically stops the current flow.

If you examine several fuses, you will see that they are rated in amperes. The fuse will melt in case the current becomes greater than the rating. Therefore a I5-ampere fuse will melt and break the circuit if the current exceeds 15 amperes

Electric circuits can also be protected with circuit breakers. A circuit breaker is a switch with a heating element о a magnetic coil in it. The heating element becomes hot i the current is too great. When the element is hot, the switch opens the circuit automatically. In the other type of breaker the circuit is broken by the pull of the magnetic coil.

Circuit breakers are made in different sizes for different circuits.

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