98 POPULAR SCIENCE MONTHLY. 



raphy, or spark telegraphy (FunTcentelegraphie) , is that of the pro- 

 duction of an effect called an electric wave or train of electric waves, 

 which can be sent out from one place, controlled, detected at another 

 place, and interpreted into an alphabetic code. Up to the present time, 

 the chief part of that intercommunication has been effected by means 

 of the Morse code, in which a group of long and short signs form the 

 letter or symbol. Some attempts have been made with more or less 

 success to work printing telegraphs and even writing or drawing tele- 

 graphs by Hertzian waves, but have not passed beyond the experimental 

 stage, whilst wireless telephony by this means is still a dream of the 

 future. 



We shall, in the first place, consider the transmitting arrangements 

 and, incidentally, the nature of the effect or wave transmitted; in the 

 second place, the receiving appliances; and finally, discuss the problem 

 of the isolation or secrecy of the intelligence conveyed between any two 

 places. 



The transmitter used in Hertzian wave telegraphy consists essen- 

 tially of a device for producing electric waves of a type which will 

 travel over the surface of the land or sea without speedy dissipation, 

 and the important element in this arrangement is the radiator, by 

 which these waves are sent out. It will be an advantage to begin by 

 explaining the electrical action of the radiator, and then proceed to 

 discuss the details of the transmitting appliances. 



It will probably assist the reader to arrive most easily at a general 

 idea of the functions of the various portions of the transmitting 

 arrangements, and in particular of the radiator, if we take as our start- 

 ing point an analogy which exists between electric wave generation for 

 telegraphic purposes and air wave generation for sound signal pur- 

 poses. Most persons have visited some of the large lighthouses which 

 exist around our coasts and have there seen a steam or air siren, as used 

 for the production of sound signals during fogs. If they have exam- 

 ined this appliance, they will know that it consists, in the first place, 

 of a long metal tube, generally with a trumpet-shaped mouthpiece. At 

 the bottom of this tube there is a fixed plate with holes in it, against 

 which revolves another similarly perforated plate. These two plates 

 separate a back chamber or wind chest from the tube, and the wind 

 chest communicates with a reservoir of compressed air or a high- 

 pressure steam boiler. In the communication pipe there is a valve 

 which can be suddenly opened for a longer or shorter time. When 

 the movable plate revolves, the coincidence or non-coincidence of the 

 holes in the two plates opens or shuts the air passage way very rapidly. 

 Hence when the blast of air or steam is turned on, the flow is cut up 

 by the revolving plates into a series of puffs which inflict blows upon 

 the stationary air in the siren tube. If these blows come at the rate. 



