=416. The Coherer.=--The coherer is a device for detecting electric waves. It consists of a glass tube with metal filings loosely packed between two metal plugs that fit the tube closely. (See Fig. 415.) These filings offer a _high_ resistance to the passage of an electric current, but when electric waves pass through the filings these _cohere_ and allow a weak current to pass through. This current may be strong enough to operate a relay connected with a sounder or bell that gives audible signals. If the tube be tapped the filings will be disturbed and the resistance again made so high that no current can pass through. =417. Wireless Telegraphy.=--In 1894 Marconi, then a young man of twenty, while making some experiments with electrical discharges discovered that the coherer would detect electrical waves at a considerable distance from their source and that by the use of a telegraph key the "dots and dashes" of the telegraph code could be reproduced by a sounder attached to a relay. At present the coherer is used principally in laboratory apparatus, as much more sensitive detectors are now available for commercial work. The essential parts of a modern wireless telegraph apparatus as used in many commercial stations are shown in Fig. 416. Alternating current at 110 volts is sent into the primary, _P_, of a transformer, the secondary, _S_, of which produces a potential of 5000 to 20,000 volts. The secondary charges a condenser until its potential becomes high enough to produce a discharge across a spark gap, _SG_. This discharge is oscillatory, the frequency being at the rate of about one million a second, depending upon the capacity of the condenser and the induction of the circuit. These oscillations pass through the primary of the oscillation transformer, inducing in the secondary, electric oscillations which surge back and forth through the antennæ, or aerial wires, _A_. These oscillations set up the "wireless waves." The production of these waves is explained as follows: An electric current in a wire sets up a magnetic field spreading out about the conductor; when the current stops the field returns to the conductor and disappears. The oscillations in the antennæ, however, have such a high frequency, of the order of a million a second, that when one surge of electricity sets up a magnetic field, the reverse surge immediately following sets up an opposite magnetic field before the first field can return to the wire. Under these conditions a succession of oppositely directed magnetic fields are produced which move out from the antennæ with the speed of light and induce electric oscillations in any conductors cut by them. [Illustration: FIG. 416.--Diagram of a commercial wireless telegraph apparatus.]