December 13, 1907] 



SCIENCE 



821 



"With ares burning in hydrogen, Mr. 

 Upson has found that the curves are gen- 

 erally much steeper for the larger values of 

 the current than for the corresponding arcs 

 burning in air. This point is of great im- 

 portance as explaining the value of the 

 hydrogenic atmosphere used by Poulsen 

 and referred to later. 



In general, I may therefore say for the 

 above arcs that increase in current through 

 the arc is accompanied by decrease of the 

 potential difference between its electrodes, 

 and vice versa decrease of the current 

 causes increase in the potential difference; 

 on the other hand certain arcs, such as the 

 are between cored carbons, behave in an 

 opposite manner, that is to say, current and 

 potential difference increase and decrease 

 together. 



I demonstrated in 1900 that if I connect 

 between the electrodes of a direct current 

 arc (or other conductor of electricity for 

 which an increase in current is accom- 

 panied by a decrease in potential difference 

 between the terminals) a condenser and a 

 self-induction connected in series, I obtain 

 in this shunt circuit an alternating current. 

 I called this phenomenon the musical arc. 

 The frequency of the alternating current 

 obtained in this shunt circuit depends on 

 the value of the self-induction and the 

 capacity of the condenser, and may prac- 

 tically be calculated by Kelvin's well- 

 known formula. 



Besides the condition that an increase of 

 current must be accompanied by a decrease 

 in potential difference, it is necessary that 

 the relative decrease in potential difference 

 produced by a given increase in current, 

 that is to say, the steepness of the char- 

 acteristic, shall exceed a certain minimum 

 value which depends on the losses in the 

 shunt circuit. It is also necessary that an 

 increase in current shall be accompanied by 

 a decrease in potential difference, even 

 when the current is varied very rapidly. 



Let us consider what takes place when I 

 connect this shunt circuit to an arc. At 

 the moment of connection a current flows 

 from the arc circuit into the condenser cir- 

 cuit, which tends to reduce the current 

 flowing through the arc. This reduction 

 of the current through the arc tends to 

 raise the potential difference between its 

 terminals, and causes still more current to 

 flow into the condenser circuit, and I now 

 have a condenser charged above the normal 

 voltage of the arc. The condenser, there- 

 fore, begins to discharge through the arc, 

 which increases the arc current and de- 

 creases the potential difference, so that the 

 condenser discharges too much; the reverse 

 process then sets in ; the condenser becomes 

 successively overcharged and undercharged, 

 due to the fact that, instead of the poten- 

 tial difference between the terminals of the 

 arc remaining constant and allowing the 

 condenser to settle down with its proper 

 corresponding charge, the potential differ- 

 ence actually decreases when the condenser 

 is discharged and increases when it is 

 charging, so as to help to keep up the 

 flowing backwards and forwards of the 

 current indefinitely. 



The oscillograph wave forms show what 

 is going on very clearly, and they show that 

 in general the swing of the current in the 

 condenser circuit attains such a magnitude 

 that when the condenser is charging it 

 takes the whole of the current away from 

 the arc, so as to make the arc, although 

 burning on a direct cui'rent, a pulsatory 

 arc. The pulsation of the current through 

 the arc causes the vapor column to grow 

 bigger and smaller, and the light to vary. 

 When the vapor column grows bigger and 

 smaller it displaces the air around it and 

 produces a note the pitch of which is de- 

 termined by the frequency of the current 

 in the shunt circuit. 



The values of the capacities of a series 

 of condensers have been calculated by 



