The magnets used to perform the previous experiments were made of steel, and incidentally it may be stated that the harder the steel, the longer will the magnet retain its strength, and also the greater the current required to first magnetize it. Soft iron, on the other hand, while becoming quickly magnetized, also quickly parts with magnetism when the exciting cause is removed.

Experiment 8.

Obtain a short piece of soft iron; the core from the magnet of an old electric bell if one is to be had, or at some blacksmith or machine shop a piece of i" Norway iron. Place the piece of iron on a piece of smooth writing paper and sift the iron filings over and around it. Observe that there is no attraction of the filings to the iron. Hold one end of the bar magnets near the piece of iron, but not touching it or the filings. It will now be seen that the piece of iron attracts the filings; that the magnet has induced magnetism in the piece of iron. By holding one of the suspended needle magnets near the piece of iron, but on the opposite end from the bar magnet, it will be observed that the iron has two poles. Test for polarity with the suspended magnet, changing the poles of the bar magnet.

Place between the bar magnet and the piece of iron a strip of thin glass. Observe that the magnetism is still retained in the piece of iron. Other non-magnetic substances such as wood, rubber, brass, etc , may likewise be placed in the same place as the glass, and yet not interfere with the magnetic induction of the piece of iron, showing that magnetism will act through all known substances with the single exception of iron. Place a strip of iron cut from a tin (tinned iron) can in place of the glass and note that the piece of iron is no longer magnetized. The strip of tinned iron acts as a screen to divert the lines of magnetic force.

The piece of iron, under the influence of the bar magnet, becomes quickly magnetized. This property is known as the permeability of the iron. Should the piece of iron have been only moderately soft, it would not entirely part with the magnetism after removing the magnet; that remainimg being known as residual magnetism, a property of value in dynamos which will be more fully considered in subsequent chapters. The steel magnet,'which holds its magnetism has what is known as retentivity. These various properties should be thoroughly studied, as they have an important bearing on the functions of many pieces of electrical apparatus and machines.