December 24, 1914] 



NATURE 



453 



■waves used for transmission are produced by three 

 arcs connected in series. Each has a negative 

 electrode or carbon only i"5 mm. diameter, and a 

 >copper disc negative electrode above it, which 

 forms part of the bottom of a cylindrical tank 

 filled with paraffin and cooled by water circula- 

 tion. The carbon electrodes are beneath this disc, . 

 and their height is regulated by means of a crank ' 

 outside the arc chamber. The arcs, which are very 

 short, burn in an atmosphere produced by mixing \ 

 acetylene and hydrogen generated from calcium 



Choker 



Adjustable 

 ResisCa,nce 



C '^Micropnones 



Choker 



Fig. I. — Diagram of connectioi.s. 



Earth 



-carbide and calcium hydride respectively, and this 

 not only prevents the burning away of the car- 

 bons, but actually increases their length slightly 



■during operation. 



A 750-\olt dynamo supplies the three arcs in 

 series, and there is means to regoilate its pres- 

 sure between 500 and 750 volts. Tw'o choking 

 -coils (to prevent high-frequency currents from 

 flowing back to the dynamo) and an adjustable 

 resistance reduce the voltage at the terminals of 

 the three arcs to between 250 and 350 volts. A 

 current of from 3^ to 4^ amperes is employed. 



Fig. 2. — The microphone and megaphone. 



The diagram (Fig. i) shows the connections 

 of the microphone and the oscillatory circuit. 

 The principal oscillatory circuit consists of an 

 inductance and variable condenser, connected in 

 parallel with the arcs A. An intermediate oscilla- 

 tory circuit B, consisting of an inductance and 

 variable condenser, is utilised to couple the prin- 

 cipal circuit with the aerial, and ensures that 

 multiple waves generated in the main circuit are 

 not transmitted to the aerial, the result being that 

 onlv a single wave is emitted. The aerial circuit 



NO. 2356, VOL. 94] 



consists of an inductance coupled with the cir- 

 cuit B and a variable condenser. A variable self- 

 induction is also used in the aerial. 



In the microphone circuit are nineteen carbon 

 microphones connected in series, and so arranged 

 that they are all acted upon by the voice 

 simultaneously by means of a megaphone. The 

 microphone and megaphone (drawn to different 

 scales) are seen in Fig. 2 ; the large end of the 

 megaphone covers the microphone. The micro- 

 phone cells are connected between the variable 

 inductance of the o.scillation transformer and 

 earth, as shown at C. This has the double advan- 

 tage of avoiding sparking, such as always occurs 

 in the microphones when they are placed directly 

 in the aerial circuit, and does not limit the energy 

 which can be taken by thcm.^ The station has two 

 complete microphone equipments, with a change- 

 over switch, so that each can be used for a short 

 time and the other microphone allowed to cool, 

 as naturally very Large currents are employed. 



The transmitting coils consist of flat spirals of 

 copper strip, and the condensers are of the glass- 

 plate type. Arrangements are provided for very 

 exact tuning by means of variable air condensers. 

 The wave-length of transmission can be varied 

 between wide limits; in the official tests a wave- 

 length of 985 metres was used. 



HIGH EXPLOSIVES IX WARFARE. 



AT the present time explosives are playing such 

 a prominent part in the war that the interest 

 and attention of the most peace-loving citizen are 

 necessarily aroused by the terrible results un- 

 doubtedly produced, or are more morbidlv aftected 

 by the tales of the alleged marvellous effects which 

 are yet to be experienced. A few^ notes on the 

 most important explosives being used in war may 

 therefore be of special interest just now. 



The explosives which can be advantageously 

 employed in warfare are by no means the most 

 powerful which the chemist can produce, or which 

 may even be used in civil engineering or mining 

 operations. The military high explosive must be 

 sufficiently insensitive to shock to prevent its 

 being exploded when struck by projectiles, or 

 when submitted to the shock of being fired from a 

 gun as the charge of shell, else it might prove as 

 dangerous to the user as to the enemy. Thus the 

 nitroglycerine class and many other explosives are 

 excluded. . 



For many years gun-cotton, containing a con- 

 siderable amount of moisture, was largely used 

 for naval and military purposes. In the moist 

 state it is extremely safe, but can be easily de- 

 tonated when a small primer of dry gun-cotton 

 is fired in contact with it. The explosive effect 

 is great, and it provided an excellent and safe 

 explosive for military mines and purposes of de- 

 struction, and as a charge for torpedoes. It was 

 not, however, suited for use in shells. 



The high explosives chiefly being used in the 

 present war for shell-filling are picric acid, trinitro- 



