Wireless Telegraph y. 301 
its purpose very well, and althogh not now used officially, will doubtless con- 
tinue to be used generally for a long time to come. 
Sir William Preece proved the practicability of his induction system by main- 
taining communication with the island of Mull during the time that the cable 
between that place and the mainland was interrupted; and I believe two of 
his installations are in use to this day. 
In the year 1864 Professor Clerk-Maxwell advanced the theory that an 
electrical discharge caused the formation of electro-magnetic waves in the 
ether, just as the vibration of a tuning fork causes the formation of sound 
waves in the atmosphere. By various scientific assumptions Maxwell was 
able to predict the action of these waves, their rate of progression, etc., but he 
never succeeded in actually producing them. 
Tt was more than twenty years later before his theory was experimentally 
confirmed by the scientist Heinrich Hertz, who, by means of his Oscillator 
and Resonator, produced and detected what have since been known as Hertzian 
waves. 
Hertz found that to produce the waves it was necessary to obtain excep- 
tionally rapid vibrations. If the prongs of a tuning fork are pressed together, 
then allowed to regain their normal position slowly, no sound is heard. To 
produce sound waves it is necessary to release the prongs of the fork suddenly. 
Similarly to produce electro-magnetic waves in the ether, the electrical dis- 
charge must be sudden. 
Hertz obtained this sudden discharge by means of an oscillator which con- 
sisted of an induction coil with the addition of two capacity areas, one being 
joined to each of the opposite poles of the coil. From each of these areas ran 
a brass rod terminating in a small knob. These rods were so arranged as to 
leave a small gap between the two knobs. The induction coil charged the 
areas with electricity, one positive and the other negative, while the air gap 
prevented their discharging across the rods. The difference of potential 
between the two areas increased, until a point was reached, when the insula- 
tion of the air suddealy broke down and allowed the areas to discharge across 
the gap by means of a spark. This spark caused a train of electro-magnetic 
waves to be propagated in all directions from the gap. 
To detect the waves, Hertz used what he termed a resonator. This con- 
sisted of a circle of copper wire, broken at one point so as to form a minute 
spark gap. This resonator was placed in the vicinity of the oscillator, the 
waves from which set up a disturbance in the circuit resulting in a tiny spark 
across the gap. This resonator, however, was not sufficiently sensitive to 
detect waves at any distance, and before Wireless Telegraphy could become 
practicable a more delicate instrument had to be found. The French scientist 
Branly supplied this want with his coherer. This consisted of two plugs of 
metal fitted tightly into a glass tube and connected to a cireuit containing a 
battery and galvanometer by means of wires led through the ends of the tube. 
The gap between the plugs was partly fitted with iron filings. In their normal 
