. FIRST EVENING DISCOURSE. 733 
electrodes, and vice versd 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 
accompanied by a decrease in potential difference between the terminals) a con- 
denser and a self-induction connected in series, I obtain in this shunt circuit an 
alternating current. 1 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 practically be calev- 
lated 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 poten- 
tial difference produced by a given increase in current, that is to say, the steepness 
of the characteristic, shall exceed a certain minimum yalue 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 are. 
At the moment of connection a current flows from the are circuit into the con- 
denser circuit, which tends to reduce the current flowing through the are. This 
reduction of the current through the are 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 are. 
The condenser, therefore, begins to discharge through the are, which increases the 
arc current and decreases 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 potential 
difference between the terminals of the arc remaining constant and allowing the 
condenser to settle down with its proper corresponding charge, the potential 
difference 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 are, so as to make the arc, although burning on a direct 
current, a pulsatory are, ‘he pulsation of the current through the are causes the 
vapour column to grow bigger and smaller, and the light to vary. When the 
vapour column grows bigger and smaller it displaces the air around it and pro- 
duces a note the pitch of which is determined by the frequency of the current in 
the shunt circuit. 
The values of the capacities of a series of condensers have been calculated by 
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 connected 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 
circuits and perform electrically some experiments which I have already performed 
mechanically earlier this evening. I use the large coil which forms the self- 
induction in the cireuit shunting the are as a transmitting circuit for wireless 
telezraphy 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 transmitting circuit. Ata certain short distance 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 connecting a condenser in it, the lamp on the receiving circuit 
