822 



SCIENCE 



[N. S. Vol. XXVI. No. 676 



Kelvin's formula to give the frequencies 

 corresponding with a musical octave, and 

 the nearest values in an ordinary laboratory 

 box of condensers have been taken and con- 

 nected to a keyboard. The result shows 

 how nearly Kelvin's law is obeyed. 



With this apparatus I can demonstrate 

 the importance of tuning in electrical cir- 

 cuits and perform electrically some experi- 

 ments which I have already performed me- 

 chanically earlier this evening. I use the 

 large coil which forms the self-induction in 

 the circuit shunting the arc as a trans- 

 mitting circuit for wireless telegraphy by 

 the magnetic induction or Preece method, 

 and I have a receiving circuit consisting of 

 a coil of wire connected to a small lamp 

 and not connected in any way to the trans- 

 mitting circuit. At a certain short dis- 

 tance between the transmitting coil and 

 the receiving coils the indicating lamp 

 lights if I cause my arc to sound any one 

 of the notes of the octave, and so produce 

 an alternating current of corresponding 

 frequency in the transmitting coil. If I 

 now tune the receiving circuit, by connect- 

 ing a condenser in it, the lamp on the re- 

 ceiving circuit will light at about five times 

 the distance ; but it will only light when 

 one definite note is sounded by the arc. 

 These are the two distinct advantages of 

 tuning, namely, greater distance and 

 syntony, or responding to only one definite 

 note. 



For wireless telegraphy by means of 

 Hertzian waves, based on my arc method, 

 we require much higher frequencies in the 

 shunt circuit. If we attempt to obtain this 

 higher frequency from the ordinary arc 

 burning between solid carbons in air, we 

 find that above a certain limit the oscilla- 

 tions will no longer take place. This is 

 due to the fact that we are varying the 

 current through the arc at this higher fre- 

 quency too quickly for an increase in cur- 



rent to be accompanied by a decrease in 

 potential difi'erence. I have demonstrated 

 that if I only vary the current through 

 the ordinary current arc sufficiently 

 rapidly, then an increase in current is ac- 

 companied by a proportionate increase in 

 the potential difference, and the arc be- 

 haves just like an ordinary resistance. If 

 we work with very small current arcs we 

 can obtain high-frequency musical arcs 

 burning in air either between carbon or 

 metal electrodes. 



In a paper read before the International 

 Electrical Congress at St. Louis in 1904 

 Mr. Poulsen showed that by placing the arc 

 in a flame it was possible to obtain higher 

 frequencies than when the are was burning 

 in air. Following this up Mr. Poulsen 

 came to the conclusion that the best results 

 were obtained when the arc was burning in 

 hydrogen, or a gas containing hydrogen ; ■ 

 and he further added a magnetic field 

 around the arc somewhat similar to that 

 which has been previously used by Elihu 

 Thomson. 



The arc burning in coal gas in a power- 

 ful transverse magnetic field was used by 

 Poulsen in his early experiments to pro- 

 duce the high-frequency current necessary 

 for wireless telegraphy between Lyngby 

 and Esbjerg in Denmark. This apparatus 

 has been further improved, and is now em- 

 ployed by the Amalgamated Radio-Tele- 

 graph Company in their station at Culler- 

 coates and the other stations that they are 

 erecting. 



In both the arc and the spark methods 

 of wireless telegraphy we employ a high- 

 frequency alternating current in the aerial 

 conductor. The essential difference be- 

 tween the two methods lies in the fact that 

 with the spark method our alternating cur- 

 rent in the aerial conductor first increases 

 to a maximum value and then dies away 

 rapidly, making only a limited number of 

 oscillations, whereas in the are method the 



