It consists of a master station in the form of a box less than a foot long and six inches deep, and any number of sub-stations that may be required. Any voice within fifteen feet is taken by the receiving instrument and carried over the wires to any distance within about a thousand miles. It has now been adopted by a great many business organizations as a convenient means of inter-communication. The Story of the Wireless Telegraph Though one or more means of transmitting messages by electricity have been known now for a great many years, the mechanisms by which they are accomplished are understood only by those who take a general interest in physical science, and the few to whom electrical communication is a profession. So far as theory and details of working are concerned, there are a good many people still in the same shadowy frame of mind as the old Aberdeen postmaster, of whom the story is told. When asked to explain the working of a telegraph instrument he said, “Look at that sheep-dog. Suppose we hold his hind-quarters here and stretch him out until his head reaches Glasgow. Then if we tread on his tail here he will bark in Glasgow. As it is not convenient to stretch a dog, we stretch a wire, and that serves the purpose.” As the name implies, “stretching a wire” is unnecessary in wireless telegraphy, though in order to understand the finer points of theory one needs to stretch the imagination a little. That, however, is not so much, because there is any inherent obscurity or difficulty in the underlying principles, as because the mechanism of all electrical effects is more or less intangible. Electricity and magnetism operate across apparently empty space, and the links which connect cause and effect have to be guessed at. Three different methods have been made use of in wireless telegraphy, which may be classed as conduction, induction and wave methods. In the first method currents are sent through the earth from an electrode to another at the sending station. In induction, use is made of the property which alternating currents possess of exciting similar currents in neighboring conductors, the aim being to get as intense a current as possible in the secondary circuit. Mr. W. H. Preece, of England, by combining the two, signaled in this way as far as forty miles. The third and the only method which has proved practically available is by the use of electro-magnetic waves.