March, 1913. 



KNOWLEDGE. 



83 



with a velocity of one thousand one hundred 

 feet per second ; therefore, the space between 

 each wave and the next will be four feet. 

 Again, suppose an electric oscillator sending 

 out one million waves per second. The 

 wave length must be three hundred metres, 

 since the velocity of propagation is JXlO 10 

 centimetres per second. 

 The frequency of an ordinary alternating current 



is far too low to 



produce 



waves 

 JV 



of 

 sufficient energy for 

 use in wireless tele- 

 graphy. In 1899 

 Duddell discovered 

 the following phe- 

 nomenon : — When a 

 condenser of suitable 

 capacity is connected 

 through a self-induc- 

 tion coil in parallel 

 with an electric arc 

 which is being fed 

 by direct current, the 

 arc will, under certain 

 conditions, become 

 musical. At the 

 same time there is 

 set up in the self- 

 induction an alterna- 

 ting current, having 

 the same rate of 

 vibration as the note 

 produced by the arc. 

 In other words, part 

 of the direct current 

 is transformed into 

 an alternating current 

 of constant ampli- 

 tude, the energy ab- 

 sorbed being supplied 

 by the direct cur- 

 rent. 



A very close anal- 

 ogy to Duddell's 

 musical arc may be 

 found in a flue organ 

 pipe. Here the note 



is produced by the vibrations of the air column in the 

 pipe, whose dimensions determine the pitch of the 

 note. These vibrations would die away, or be 

 damped, very rapidly but for the energy supplied by 

 the continuous current of air blown across the 

 embouchure of the pipe. This air current is directed 

 against a sharp edge, and must pass either inside or 

 outside it, thus creating either a compression or a 

 rarefaction at the end of the pipe. The natural 

 frequency of the pipe causes the air current to be 

 turned alternately inwards and outwards, and the 

 note is maintained, the air current supplying the 

 energy and paying the piper, the organ pipe calling 

 the tune. 



Fio'jt a photograph 



A Tv 



By this method Duddell obtained a high rate of 

 alternation : that is to say, up to thirty or forty thou- 

 sand oscillations per second. Even this rate is too low 

 for wireless telegraphy, and the energy far too feeble. 

 In 1903, however, Professor Yaldemar Poulsen, of 

 Copenhagen, succeeded in obtaining a much higher 

 rate of oscillation by surrounding the arc with an 

 atmosphere containing hydrogen. Hydrogen, with 

 its high atomic velocity, possesses great thermal and 



electrical conductiv- 

 ity, and no doubt 

 its cooling influence 

 makes it possible to 

 use higher power. 

 Poulsen further dis- 

 covered that a strong 

 magnetic field placed 

 transversely to the 

 arc had the effect of 

 giving a definite shape 

 to the arc and thus 

 making the oscilla- 

 tions more constant, 

 as well as making it 

 possible to increase 

 the potential differ- 

 ence greatly in pro- 

 portion to the length 

 of the arc. By these 

 two devices Poulsen 

 has obtained alterna- 

 tions as rapid as one 

 million per second. 



It is claimed for 

 the Poulsen-Pedersen 

 system of wireless 

 telegraphy that it is 

 capable of being 

 tuned with greater 

 precision than any 

 spark system. If a 

 tuning fork in vibra- 

 tion is held over an 

 open piano, only one 

 i v w. DuddtiL wire will show reson- 

 ance. On the other 

 hand a pistol shot 

 will make all the 

 wires vibrate at the same time. 



Another claim is that a speed comparable with 

 that of submarine cables can be attained by the 

 Poulsen automatic transmitter (Figure 74;. With 

 this, as in the Wheatstone instrument, the message 

 to be sent is represented by a series of holes punched 

 in a continuous paper strip, those on one side of 

 the central line representing dots, and those on the 

 other side dashes. This strip is fed into a rotating 

 contact maker, and by means of the holes the con- 

 tinuous waves of the arc are cut up into lengths, 

 short for the dots and long for the dashes. The 

 aerial wire at the receiving station picks up these 

 wave trains, and transmits them to a crystal rectifier 



Figure 76. 

 3 Kilowatt Generator. 



