508 Mr. W. Duddell on High Frequency Currents. [May 17, 1912 



arc by itself is essentially unstable. Of course, in practice, the 

 current can be kept at any desired value by putting sufficient resist- 

 ance in series with the arc, so that when the current is increased the 

 potential difference between the terminals of the arc and the resist- 

 ance taken together increases, and the circuit as a whole becomes 

 stable. If such an arc be shunted with a capacity and self-induction 

 in series (see Fig. 13) oscillations will be set up in this shunt circuit, 

 the frequency of the oscillations being mainly determined by the 

 periodic time of the shunt condenser circuit, which may be calculated 

 from the well-known Kelvin formula. 



When the oscillations are taking place the oscillatory current 

 flows through the arc, and consequently tends to increase and 

 decrease the arc current. Any increase in the current tends to make 

 the vapour column of the arc larger, and any decrease makes it 



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M/ 



Fig. 13. — Musical Arc Connections. 



smaller, so that the vapour column of the arc pulsates in size. This 

 pulsation in the size of the vapour column moves the air in the 

 neighbourhood of the arc, and, if the frequency is suitable, produces 

 sound ; hence this experiment is known as the musical arc. By vary- 

 ing either the capacity or the self-induction the frequency of the 

 oscillations can be varied, and hence the pitch of the notes given out 

 by the arc, and by this means a tune can even be played on it. 



The frequency of the currents produced when using an ordinary 

 arc between soHd carbon electrodes is comparatively low, at the most 

 a few thousands per second. The reason for this is that the 

 characteristic of the arc — that is to say, the relationship between the 

 potential difference and the current— depends to some extent on how 

 quickly the current is varied. If the current is varied very quickly, 

 then an increase of current is accompanied by an increase in the 



