What Is an Electric Current?
By an electric current, is meant a flow of electricity along a conductor. The flow of electricity, through your body, when you receive an electric shock, is a current, but it lasts only for an instant, and it is difficult to learn much about its nature. By the use of various devices, it is possible to produce currents, that will continue as long as we want them, so that we are enabled to study their properties quite thoroughly.
One of the oldest and simplest forms of apparatus, for producing electric currents, is that which is known as the voltaic cell. This form of apparatus may very easily be constructed. Pour some water into a glass jar, and add a little sulphuric acid. Now place in the water a strip of clean zinc and one of clean copper. Do not let the strips of metal touch in the water, but connect them outside the water by means of a piece of wire. When this has been done, a current of electricity will be sent up along the wire and through the water between the two strips of zinc and copper. This current is said to flow along the wire from the copper, which is called the positive pole of the cell, to the zinc, which is called the negative pole. In the liquid in the cell (i.e., the jar), the current travels from the zinc to the copper, thus completing what is called the electric circuit. Whenever the circuit it broken, that is, whenever there is a gap made in the wire connecting the poles, or anything else is done to destroy the completeness of the path, along which the current travels, the current ceases; consequently, when it is desirable to stop the current, all that is necessary is to cut the wire connecting the two strips of copper and zinc.
The production of a current of electricity, by means of an apparatus of this sort, depends upon the chemical action of the acid in the water upon the strip of zinc. As long as the acid continues to act upon the zinc, the current is produced, and when the acid ceases to act upon the zinc, the current ceases to flow. If the zinc is clean, the chemical action of the acid ceases, whenever the circuit is broken, and consequently, when the cell is not being used to produce a current, the zinc is not destroyed by the acid. But if the zinc is not clean, small electric currents are set up, within the liquid, between the zinc and the impurities on its surface, and around the points where these impurities lie the acid acts upon the zinc and dissolves it. This action of the acid upon the zinc, when the circuit is broken, is known as local action, and it is very desirable to prevent it, as far as possible. For this purpose the zinc is often rubbed with mercury, which soaks into the zinc and forms a film on its surface, upon which the impurities float. This treatment of the zinc is known as amalgamation, and it serves to prevent almost all the local action, due to impurities of the zinc.
Many other substances, besides zinc and copper, have been found capable of yielding an electric current, when placed in a suitable liquid, and many other fluids, besides water that contains a little sulphuric acid, have been employed to act upon the zinc and copper, or the substances used in their stead. Numerous cells of different kinds have, therefore, been devised, but, in all of them, the current is produced by chemical action. Most of them contain a liquid of some sort, which is called the exciting fluid, and two solid substances, which are called the elements of the cell. One of these elements is always much more susceptible to the chemical action of the exciting fluid, than the other, and this one is known as the positive element. The other element, upon which the exciting fluid may have no action, is called the negative element. In cells in which the elements are zinc and copper, the zinc is always the positive element. This may seem strange to you, for you have already learned that the zinc is the negative pole of the cell, but, to avoid confusion, you must fix well in your mind the fact that the zinc is not the positive element of a voltaic cell, but its negative pole, and that the copper, which forms the negative element is the positive pole of the cell. The currents produced by the various forms of voltaic cells, vary considerably in strength, but none of them are very strong. In order to obtain a stronger current, a number of cells must be used together. Such a collection of cells forms a voltaic battery, and in some instances, as many as fifty thousand cells have been used in a single battery.
We have already learned in our study of water that it may be separated into its elementary gases by sending an electric current through it. The effect is a chemical one. Water, however, is not the only substance that is decomposed by electricity; almost all chemical compounds may be decomposed by the passage of a current through them, provided a current of sufficient strength is used.
Another effect of the current is its heating effect. It has been found that the passage of an electric current, through any body, is always productive of a certain amount of heat. The amount of heat produced depends upon the strength of the current of electricity, and the resistance to its passage that is offered by the body through which it travels. This amount is increased by increasing either the strength of the current or the resistance of the conductor along which it travels. We have already learned, that some substances allow electricity to pass over them very readily, and are therefore called conductors, while substances through which electricity does not flow readily are known as non-conductors. No substance is a perfect non-conductor, for electricity can be made to pass through any substance, if the current is sufficiently powerful. Neither is any substance a perfect conductor, for all substances offer some resistance to the passage of an electric current. Those substances that are ordinarily considered good conductors offer varying degrees of resistance to electric currents. For example, a copper wire offers less resistance than an iron wire of the same length and diameter.
The resistance of a body depends not only upon its material, but also upon its length and size. In conductors of the same material, the resistance is directly proportional to the length of the conductor, and inversely proportional to the square of its diameter. This is not surprising, for an electric current bears a strong resemblance to a current of water, in many of its properties, and you know that it is harder to force water through long, narrow pipes, than through short, wide ones.
From what has been stated about resistance, you may see, that a current will produce more heat, in passing through a long fine wire, than through a shorter and thicker one, and that, of two conductors of the same length and size, but of different material, one may be heated much more by a current than will another.
HOW MAGNETS ARE MADE
A third effect of the electric current, which has not previously been mentioned is its magnetizing effect. It is upon this, that some of the most important effects of electricity depend.
By coiling a wire around a bar of iron or steel, and then sending an electric current through it, the piece of iron, or steel, is made to show magnetic properties. By this is meant, as you doubtless know, that the iron will now attract other pieces of iron, or steel, to it. The strength of this attraction depends upon the strength of the current, and upon the number of turns of wire around the bar. By increasing either the strength of the current, or the number of turns in the coil of wire, around the bar of iron, the strength of its magnetic attraction is increased. When the current is stopped, the magnetic properties of the iron disappear almost completely. A magnet, that depends upon a current of electricity for its magnetic power, is called an electro-magnet.
Besides electro-magnets there are others, which are called permanent magnets. Electro-magnets are composed of soft iron, the softer the better, and, as soon as the current of electricity ceases to flow around them, their magnetic properties disappear. Permanent magnets, on the contrary, are made of steel, and their magnetism is independent of the action of a current of electricity. No coil of wire is wound around them, and no current is employed to maintain their magnetic properties. A piece of steel may be made to become a permanent magnet, by passing a current of electricity, for a considerable time, through a coil of wire wound around it, or by allowing a piece of steel to remain for some time in contact with a strong magnet. When a current of electricity passes through a coil of wire, wound around a bar of steel, it takes longer to magnetize the steel than it would to magnetize iron, but, when the current ceases, the magnetism does not all disappear from the steel. A portion of it remains, and the steel becomes permanently magnetic.
If a thin bar of steel is magnetized, and is then suspended by its middle, so that it can spring freely, it will be found that one end tends to point toward the north, and the other toward the south. Whenever the bar is swung out of this position, it swings back to it, and if the north end is turned entirely around to the south, it does not remain, but swings back to its former position. This shows that there is a difference in the magnetism at the two ends of the magnet. To indicate this difference, the north-seeking end of a magnet is called the positive pole of the magnet, and the south-seeking end is known as the negative pole.
By suspending two bar magnets, in the manner described, it can be shown that the positive and negative poles of the magnets act like positive and negative charges of electricity. Poles of the same kind repel, and poles of opposite kinds attract, each other.
Permanent magnets are usually made in two forms, either straight or horseshoe shaped. A compass needle, as has been shown, is an example of a straight magnet. The horseshoe variety, which has a little bar of iron, called the keeper, laid across the poles is a common toy. Electro-magnets are seldom seen, except in electrical instruments or machinery. The pictures shown on the following pages give us a bird’s-eye view of some of the wonders performed by these electro-magnets. Tons and tons of material are picked up and held securely by one of these magnets as easily as you can hold on to an apple.