October 31, 1912] 



NATURE 



scientific phenomena are imperfectly understood. If 

 we are to overcome present difficulties and limitations 

 and make fresh advances, it can only be by a thorough 

 comprehension of the physics of wireless telegraphy. 

 Hence it will be more to our advantage to bring com- 

 bined scientific thoughts to bear upon the matters on 

 which even leading experts difl'er or are ignorant, 

 rather than let our symposium resolve itself into a 

 discussion on apparatus or systems or the recitation 

 of performances and the record of results. 



As the only type of wireless telegraphy which has 

 any considerable theoretical interest at the present time 

 is that involving the application of unguided electro- 

 magnetic waves, our attention will doubtless be chieflv 

 directed to it. 



Starting from the discoveries of Hertz and his fol- 

 lowers, we enter a new era. Apart from Marconi's 

 improvements in the metallic filings coherer of 

 Hughes, Branly, and Lodge, the important element 

 in the arrangements by which in 1896 he applied 

 purely scientific knowledge of Hertzian electric waves 

 to practical electric waves or radio-telegraphy was the 

 introduction of the long, nearly vertical aerial wire, 

 as .-1 radiator combined with a metal plate above or 

 buried in the earth as the balancing capacity. In 

 this wire high frequency oscillations are created ; 

 originally by using the wire itself as one electrode 

 of an air condenser, and the earth as the other, but 

 later on by inducing oscillations in the wire by means 

 of the dead-beat or oscillatory discharge in another 

 condenser circuit including a spark gap, coupled to 

 the air-wire circuit. Although enormous ingenuitv 

 has been expended in improving or varying every 

 element in the appliances, we can say that with the 

 exception of a small number of stations using the 

 Duddell-Poulsen arc generator, nearly all the practical 

 wireless telegraphy in the world is at present (1912) 

 conducted by the following apparatus. 



.At each station there is a transmitter which com- 

 prises three elements : — 



1. .A source of high electromotive force which mav 

 be a continuous-current dynamo and storage batterv, 

 an alternator and transformer, or a batterv and in- 

 duction coil giving continuous, alternating, or inter- 

 rupted high-tension electromotive force. 



2. A condenser in which the generator stores an 

 electric charge to be suddenly released when a certain 

 potential is attained across a spark gap in the form 

 of an electric discharge passing through a coil in 

 series with the condenser. 



3. An open or radiative circuit coupled to the con- 

 denser circuit, comprising an antenna or arrangement 

 of elevated air wires, a balancing capacity or counter- 

 poise often buried in the earth, the two being con- 

 nected through an adjustable inductance coil. 



At the receiving station we have also three 

 elements : — 



1. .An absorbing antenna by which the radiation 

 from the transmitter is picked up, creating in it high- 

 frequency oscillations. 



2. A condenser circuit having variable capacity and 

 inductance coupled to the antenna and syntonised to it. 



';. .Some form of oscillation detector connected in 

 series or parallel with the above condenser which is 

 affected by the oscillations and sets in operation a 

 recordinar or indicating device which makes a visible 

 or audible signal. 



Generallv speaking, at any one station the radiatinS 

 and .nbsorbing antennas are one and the same, and 

 used for both purnoses alternatelv, and each station 

 has both transmitting and receiving' apparatus. The 

 functions are, however, not identical. What is re- 

 quired in the transmittino- antenna is a certain hein'ht 

 and also free or insulated ends. In the receivine 

 antenna, pot onlv heitrht b'lt surface is required, 



NO. 2244, VOL. 90] 



although this antenna can be laid parallel with and 

 close to the earth and earthed at both ends ; but pro- 

 vided it is half a wave length in length, it will still 

 absorb a considerable amount of energy from electric 

 waves arriving in its own direction. - 



In the next place as to exact details, the following 

 information may be useful to those who are not 

 wireless-telegraph engineers. 



The antenna consists of a large number of hard- 

 drawn copper wires, which are upheld by masts or 

 towers in such fashion that the wires form a sort of 

 fan elevated in the air ; or they may rise up for a 

 ceitain height and then be bent downward on all sides, 

 like the ribs of an umbrella. In the case of our 

 battleships, they are groups of parallel wires kept 

 separate by wooden stars and stretched between the 

 masts and then led downwards to the bow and stern 

 of the ship. In the high-power Marconi stations they 

 rise up vertically for a certain distance, and are then 

 stretched horizontally for a distance about five times 

 greater, parallel with the ground. 



In long-distance stations the wooden or steel lattice 

 towers or tubular masts required to sustain these 

 wires are elaborate structures 100 to 400 feet or more 

 in height, and have to be well stayed to resist wind. 



Associated with the antenna is a counterpoise or 

 balancing capacity, which may consist of insulated 

 wires stretched a little way above the earth, or 

 radiating wires or metal nets laid in the earth, or 

 sheets or nets of metal laid on the ground, or even 

 the metal hull of a ship. 



This counterpoise is connected to the antenna 

 through a variable inductance coil. In virtue of the 

 capacity of the antenna with respect to the earth or 

 the counterpoise, the whole ' system has a natural 

 time period of electrical oscillation. 



It may be compared with an elastic steel strip held 

 at the bottom iri a vice and loaded at the top, which 

 can be set in vibration by small blows administered to 

 it at the proper rate. 



There are certain rates of antenna oscillation re- 

 served for certain purposes. 



Thus, for ship or coast signalling, antennae are 

 used having natural time periods of one-millionth or 

 one half-millionth of a second, and for large power 

 stations the time period may be as large as one 

 hundred-thousandth or one fifty-thousandth of a 

 second. 



In nearly all cases these oscillations are excited in 

 the antenna by the intermittent discharge of a con- 

 denser. They are therefore damped or decadent trains 

 of free oscillations, separated by intervals of silence. 

 The group frequency, as it is called, or number of the 

 trains of oscillations, is now usually 500 to 1000, since, 

 when using the telephone as a receiver, the group 

 frequency is preferably that frequency for which the 

 telephone is most sensitive. Each train of oscilla- 

 tions may comprise 30, 50, or 100 oscillations having 

 the antenna frequency. The antenna is, therefore, set 

 in electrical vibrations, so that trains of electric 

 currents run up and down it intermittently, say, 500 

 times a second, each train consisting of 50 or more 

 decadent oscillations, whilst each oscillation or single 

 current occupies a time between one fifty-thousandth 

 of a second and one two-millionth of a second and its 

 complete to and fro cycle. 



These high frequency currents in the antenna are 

 created by the induction of a nearly dead-beat or else 

 an oscillatory discharge of a condenser. In small 

 installations the condenser is a collection of Leyden 

 jars, or, more conveniently, glass plates coated with 



2 Numerou.s patents have beetl taken out for methods of usin^ an antenna 

 at the same t'me for isending and receiving. The inventions of Mr. Marconi 

 in connection with this matter are both practical and important, and are 

 being carefully developed by him. 



