We have tried to describe to you the general principles underlying the art of wireless telegraphy as plainly as possible, using for illustration the simplest kind of apparatus employed for the practical sending and receiving of messages. At the present day there are several systems in actual practice, and with the growth of the art there have been many elaborations of apparatus that have come into use. For instance, the coherer is not as much used as formerly. In its place there are employed several kinds of "wave-detectors" as they are now termed, and in many of the systems the electrical pulsations are generated by a dynamo-machine instead of batteries. Then, again, instead of the messages being recorded by a Morse register at the receiving end, the operator receives them by means of a telephone receiver, through which he hears the Morse characters and writes them down in words as he hears them. Generally the aerial, or "antennæ," as it is sometimes named, consists of several wires, sometimes a large number, carried to a considerable height. There are a great many other details which might be written to explain all the complicated apparatus which is used in some of the systems, but it is not intended in this book to offer more than a general explanation of main principles. We must leave it to you to study the details elsewhere if you so desire after you have read these pages. VI THE TELEPHONE You probably all know that the telephone is an electrical instrument by which one person may talk to another who is at a distance. Not only can we talk to a person who is in a different part of the city, but such great improvements have been made in these instruments that we can talk through the telephone to a person in another city, even though it be hundreds of miles away. The main principle of the telephone is electromagnetism, as in the telegraph, but there are other important points in addition to those we mentioned in describing the latter. Let us take first the INDUCTION-COIL You will remember that an electromagnet is made by winding many turns of wire around a piece of iron and sending a current of electricity through this wire. Now, suppose this current of electricity was being supplied by two cells of a battery. If you took in your hands the wires coming from these _two cells_, giving, say, four volts, you could not feel any shock; but if you were to take hold of the ends of _the wires_ on the _electromagnet_ and _separate_ them while this same current was going through, you would get a decided shock.