When an arc is properly connected with a condenser and an impedance coil it will emit a musical note. The note is due to rapid changes in the arc, a very important factor which led to its recognition as a a value in wireless telephony.
FIG. 144.—How the sound waves of the voice are impressed upon undamped oscillations.
When the condenser and inductance are shunted across an arc supplied with direct current, the condenser immediately becomes charged, and the current through the arc is diminished. The potential difference across the latter is therefore instantly increased, tending to further charge the condenser. This increase of charge reacts on the arc, increasing its current. The condenser discharges, through the inductance coil, and becomes charged in the opposite direction, just like a spring, which released, goes beyond its normal position and then returns.
The operation is repeated many times per second (usually over 1,000,000), setting up persistent undamped oscillations.
FIG. 145.—Arrangement of the speaking arc.
Perhaps a better conception of how it is possible for a continuous current, such as that which supplies the arc, to change into alternating current, vibrating backward and forward, may be gained by comparison with the steady forward motion of a violin bow, which produces a to and fro motion of the strings.
FIG. 146.—Diagram showing how a wireless telephone transmitting system is arranged.