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In order to avoid repetition and confusion and to make each instrument or principle which has been discussed stand distinctly by itself, the text has been divided into separate lessons following in their arrangement, as far as has been possible, the logical sequence.

For the same reason, and also because of lack of space all details pertaining to the actual maintenance and adjustment of the instruments has been embodied in another book called "The Operation of Wireless Telegraph Apparatus."

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Natural Magnets. Artificial Magnets. Magnetic Field of Force.

It was known to the ancients that certain hard, black stones, an iron ore consisting of iron and oxygen found notably at Magnesia in Asia Minor, possessed the power of attracting small pieces of iron or steel. This almost magic attribute of the stone was early turned to account in navigation and secured for it the name of Lodestone (leading-stone) because of its remarkable property of pointing north and south when suspended by a thread. The name of magnet (magnes lapis) was also given to these stones.