810 



SCIENCE 



[N. S. Vol. XLII. No. 1093 



Fig. 2 represents the simplest form of 

 wireless transmission. An electrical gen- 



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EARTH 



erator at A sends through a vertical wire 

 electrical energy to a vertical wire at sta- 

 tion B. There is motion of electricity be- 

 tween A and B, but only through the earth. 

 The second case is similar to the first ex- 

 cept that in the second ease there is no wire 

 G, connecting station A to station B; hence, 

 on account of the absence of the connecting 

 wire G, we call the second method of trans- 

 mission a "wireless" method. This second 

 method is particularly important when it is 

 impossible to employ a connecting wire be- 

 tween the two stations, as, for instance, 

 between two ships at sea, or between a ship 

 and the shore. 



This more or less insignificant difference 

 in the structures, by means of which we 

 transmit, necessitates, however, the em- 

 ployment of almost radically different elec- 

 trical actions in order to transmit energy 

 from A to B. Whereas in the first case we 

 can transmit from A to B any reasonable 

 amount of energy by a constant or a slowly 

 varying motion of electricity, we have to 

 adopt in the second case a very rapidly oscil- 



aI B 



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Fig. 3. 



lating motion of electricity. The simplest 

 and historically the oldest method of pro- 

 ducing a rapidly oscillating motion of elec- 

 tricity was obtained as follows : The vertical 

 wire at the transmitting station A, called 



the antenna, has an air gap cd, and the two 

 parts of the antenna, the upper part which 

 is insulated, and the lower part which is 

 connected to the earth, are connected by 

 means of wires a and & to a very high elec- 

 trical tension such as is employed in our 

 automobiles for ignition or in the produc- 

 tion of X-rays by means of the X-ray tubes 

 and the induction coil. This high electrical 

 tension forces one kind of electricity into 

 the upper part of the antenna and the oppo- 

 site kind into the lower part of the antenna 

 which is connected to the earth. The two 

 parts of the antenna form the two conduct- 

 ing coatings of a Leyden jar; the surround- 

 ing atmosphere, of which the air gap cd is 

 a part, separates the two coatings. "When 

 the electrical tension is very high it breaks 

 through the air space cd, that is, a spark 

 jumps between the two metal balls cd and 

 forms there a conducting path, that is an 

 easy path for the motion of the electricities 

 which are separated, one crowded into the 

 upper part of the antenna, and the other 

 into the lower part and the earth. These 

 two separated electricities which attract 

 each other will rush toward each other as 

 soon as the passage through the air gap cd 

 has been established, and they will move as 

 fast as the laws of motion of electricities 

 command them to do. Now these laws de- 

 mand that this motion be an oscillatory one. 

 This oscillatory motion of electricity during 

 a discharge of a Leyden jar was discovered 

 by our great Joseph Henry in 1840 when 

 he was professor of physics at Princeton 

 College, and the laws of motion were first 

 formulated in 1855 by the famous WiUiam 

 Thomson, who died a few years ago as Lord 

 Kelvin. The oscillatory motion of electri- 

 city and the laws governing it can be best 

 illustrated by the following simple mechan- 

 ical analogy. A stiff steel tongue db which 

 is fastened at its lower end a to a table is 

 displaced by the tension of a string d from 



