=425. The Wireless Telephone.=--One of the most important developments in wireless communication in recent years has been in wireless telephony. We realize its possibilities, when we hear of the achievements of talking across an ocean or between airplanes and the ground. The wireless telephone can be best understood by comparing it with the common telephone. When the latter is in use, a direct current flows continually through the instrument. (See Arts. 312-316.) When a person speaks into the transmitter, the sound waves of the voice cause the diaphragm to vibrate, this action causes rapid changes in the _resistance_ of the transmitter, which in turn causes the direct current to fluctuate just in step with the pulses of the voice waves. This fluctuating direct current passes through the primary of an induction coil, producing in the secondary an intensified alternating current. This passes over the line wires to the receiver where it produces variations in the magnetic field affecting the receiver diaphragm, causing the latter to reproduce the voice of the person speaking in the transmitter. Now to make the comparison clear, two facts must be noted with regard to the wire telephone: first, there must be an action in the transmitter which causes variations in a current through the instrument; second, this fluctuating current produces a more intense alternating current which flows over the line and affects the receiver diaphragm, producing there sound vibrations of greater intensity than those used at the transmitter. This added energy comes from the current flowing through the transmitter. The case is analogous to that of an electric bell. The armature of the bell vibrates with greater energy than is required to push the button, the extra energy being derived from the battery.