The bells and magnetos are for the purpose of calling the central operator or the person at the other end of the line and drawing attention to the fact that some one wishes to get into communication with him. The older styles of telephones used what is known as a polarized bell and a hand magneto for this purpose. A polarized bell is a very sensitive piece of apparatus which will operate with very little current. A magneto is a small hand dynamo which when turned with a crank will generate a current causing the bell at the other end of the line to ring. When the telephone receiver is raised off its hook in order to place it to the ear the bell and magneto are automatically disconnected from the line and the receiver and the transmitter are connected in their place. The current necessary to supply the telephone and receiver is supplied by two or three dry cells placed inside of each telephone. The latest types of instruments employ what is known as the central energy system, wherein the current is supplied by a large storage battery located at the central office and serving as a current supply to all the telephones connected to that system. It would be impossible to enter into the history of the telephone far enough to explain the details of some of the various systems in every-day use in such a book as this because of the immense amount of material it would be necessary to present. Such a work would occupy a volume of its own. Additional information may be readily found in any reference library. However, the "boy electrician" who wishes to make a telephone for communicating between the house and barn, or with his chum down the street, will find the necessary information in the following pages. If this work is carried out carefully and a home-made telephone system built and installed it will not only prove a very interesting undertaking but will also serve to dispel all mystery which may surround this device in the mind of the young experimenter. How to Build a Telephone Telephone receivers are useful for many purposes in electrical work other than to receive speech. They are used in connection with wireless instruments, in place of a galvanometer in measuring electrical circuits, and for testing in various ways.