October 14, 1915] 



NATURE 



183 



the size of an object as seen in a small telescope by 

 moving- away from it a certain distance. Thus, 

 suppose that a man was seen at a distance of 200 yd., 

 or boo ft., then his apparent height would be covered 

 by the width of a pencil held about 2 ft. from the 

 eye. Suppose the observer were to approach to half 

 that distance or move in 300 ft., then the apparent size 

 of the man would be doubled. If, however, the man 

 were a mile away, then moving towards him 100 yd. 

 would only increase his apparent height by about 6 

 per cent. Hence we can determine the distance of 

 an object by finding out how much the apparent size 

 is increased when we move in towards it 100 yd. or 

 any assigned distance. ' • 



Another marvellous application of science in war is 

 that of wireless telegraphy in connection with aero- 

 planes and airships as a means of scouting and rapid 

 communication of intelligence. 



The difficulties connected with it are, however, con- 

 siderable, and it has greater limitations than the un- 

 initiated would suppose. 



In the case of aeroplanes the first of these is the 

 weight of the apparatus. The military aeroplane is 

 already loaded to its fullest extent. In addition to the 

 pilot and observer and the bomb ammunition, it carries 

 in nearly all cases some gun equipment. Hence any 

 wireless apparatus must be made as light and compact 

 as possible. A wireless transmitter of the so-called 

 spark type involves three elements : (i) some source 

 of electromotive force such as a battery or dynamo, 

 (ii) an induction coil or transformer for creating a high 

 electric potential or pressure, and (iii) some form of 

 condenser or Leyden jar which is charged and then 

 discharged across a spark gap, thus creating rapid 

 movements of electricity called electric oscillations. 

 These oscillations are then caused to create others in 

 a long wire called the aerial wire. 



In the case of aeroplanes and airships the source of 

 electromotive force is generally a small dynamo or 

 alternator, which is coupled to the engine, and the 

 voltage or pressure is raised to 30,000 volts or so Dy 

 a small transformer sealed up in oil in a box. The 

 condenser consists of metal plates sandwiched between 

 sheets of glass or ebonite, and the spark balls between 

 which the spark passes are also enclosed. The weight 

 of the whole apparatus has to be kept below 100 lb., 

 and such apparatus has been designed having a weight 

 of not more than 30 lb. The French use a set weigh- 

 ing about 70 lb. One of the difficulties is to dispose 

 the aerial wire conveniently and safely. It is some- 

 times made of aluminium and stretched on insulators 

 carried by light supports on the wings, but the diffi- 

 culty is to obtain in this way sufficient length. One 

 plan adopted is to coil the wire on a reel, which the 

 observer can uncoil and let it float out behind the 

 aeroplane. 



The wire must be connected to the reel by a safety 

 catch so as to be released at once if it catches in trees 

 or buildings. By this means an aerial wire of 100 ft. 

 in length can be employed. The observer has near 

 his hand a key by which he controls the spark dis- 

 charges and so sets up in the aerial wire groups of 

 electric oscillations which create electric waves in the 

 aether, and signal the message in Morse code. 



In this manner there is not much difficulty in equip- 

 ping aeroplanes with transmitters which will send 

 messages 30 miles or so to a corresponding earth 

 station. 



These latter are the military portable motor-car or 

 pack stations, the details of which were described in 

 a lecture given here last year on " Wireless Telegraphy 

 in War." 



The receiving arrangements used on aeroplanes 

 comprise a head-telephone which is worn by the ob- 

 server associated with some simple form of detector 

 NO. 2^98, VOL. q61 



such as a carborundum crystal, aided by which the 

 observer hears the signals sent to him in Morse code 

 as long and short sounds in the telephone. 



The noise of the aeroplane engine and that of the 

 rush of air renders this method of aural reception a 

 matter of great difficulty, especially as the messages 

 must be sent in secret code, and the observer must 

 |therefore hear every letter distinctly if the message 

 IS to be intelligible. Great efforts have been made to 

 devise methods of reception which shall appeal to the 

 eye by a visual signal rather than to the ear, but the 

 exceedingly small electric currents set up in the aerial 

 wire by the arriving waves make this a matter of 

 extreme difficulty, and the problem has not yet been 

 completely solved. There is then the difficulty caused 

 by '"jamming." If the signals from an aeroplane are 

 picked up by a hostile station, this latter at once 

 sends out powerful but unmeaning signals the object 

 of which is to blur and drown out the reception or 

 sending of signals by this aeroplane. Moreover, the 

 sending of wireless signals by an aeroplane reveals 

 its presence to hostile earth stations before it can be 

 seen by the eye. 



Hence wireless telegraphy may be a means of reveal- 

 ing the enemies' scouts, and it involves a certain kind 

 of war in the aether as well as war in the air. 



In the case of airships there are other difficulties as 

 well, and it is interesting to note that there are special 

 difficulties in connection with Zeppelins. These aerial 

 monsters are, as everyone knows, constructed with a 

 framework of aluminium, containing in its interior 

 the eighteen or twenty balloons inflated with hydrogen. 

 Now as we rise upwards in the air the electric potential 

 increases rapidly, and if a conducting body at a height 

 gives off water drops or products of combustion, it is 

 rapidly brought to the potential of the air at the place 

 where it is. In the case of Zeppelins this equalisation 

 is no doubt brought about by the escape of products 

 of combustion produced by the engines. When the 

 conducting body is brought down suddenly to earth 

 again, there may be a great difference of potential 

 between it and the objects on the earth. If it is a 

 good conductor, a spark may pass, and if it is, as in 

 the case of a Zeppelin, a conducting body containing 

 a highly inflammable gas, leakage of which cannot 

 altogether be prevented, this spark may cause an ex- 

 plosion and destruction of the airship. Again, the 

 violent electric oscillations created in all metal objects 

 near powerful radiotelegraphic apparatus may cause 

 sparks to jump between metal parts, and hence may 

 inflame a hydrogen leak. 



It has therefore been recognised that there are 

 special electrical difficulties in connection with the 

 working of wireless on rigid airships with metal 

 frames and also in connection with the use of spark 

 apparatus. However carefully the actual working 

 spark is enclosed there is always risk of induced 

 sparks. 



There is room, therefore, yet for much research and 

 experimenting in connection with the use of wireless 

 telegraphy on aeroplanes and airships, and the prac- 

 tical problems are by no means completely solved. 



This leads to the consideration of the methods we 

 have adopted for dealing with these and all other 

 suggestions of the same kind of the nature of war 

 inventions. 



The Royal Society appointed certain committees at 

 an early stage in the war to deal with engineerings 

 or mechanical and with chemical inventions. Thes* 

 committees were constituted secret committees, andE 

 none of the fellows except the council and the small! 

 number of the appointed fellows were allowed eveii 

 to know the names of the members. The ostensiblej 

 reason for this unusual secrecy was that the com- 

 mittees should not be inundated with correspondence! 



