174 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
° 
potential difference of the carbons. The continuous are with solid 
carbons was said therefore to have a negative resistance.“ 
The explanation of the manner in which the continuous current 
arc maintains undamped oscillations in the condenser circuit is then 
as follows: If a condenser and inductance are shunted across the are, 
the condenser begins to be charged, and this robs the arc of some 
current. This change, however, raises the potential difference of the 
carbon poles and the charging of the condenser therefore continues. 
When the condenser is full the are current is again steady. The 
condenser then begins to discharge back through it, and this increases 
the current through the are and therefore decreases the potential dif- 
ference of the carbons. The condenser therefore continues to dis- 
charge. The action resembles that by which the vibrations of the 
column of air in an organ pipe controls the behavior of the jet of 
air from the mouth which impinges against its lip, forcing the jet of 
air alternately into and outside the organ pipe, and so maintaining 
stationary oscillations in it. The jet of air from the mouth of the 
pipe corresponds to the continuous current arc, the closed or open 
pipe associated with it is a resonant circuit and corresponds with the 
condenser and inductance. 
Consider the state when the oscillations have been set up in the 
condenser circuit. We must assume that there is a stream of elec- 
trons from the negative terminal of the arc making their way across 
the interspace to the positive terminal. If, then, we consider the 
state at the instant when the condenser has reversed its charge, so that 
the coating connected to the negative are terminal is positively 
charged, we see that there is a tendency for the stream of electrons 
to enter the condenser and supply the deficiency represented by the 
positive charge on that plate. They are, so to speak, sucked into the 
condenser. Accordingly this action either annuls or reduces the cur- 
rent in the are. When the condenser is charged to the potential 
difference then existing between the terminals of the arc, no more 
electrons enter it, and they then all travel across the are. This in- 
crease in the are current is accompanied by a fall in the electronic 
density difference, or the potential difference of the arc terminals, 
and the condenser then begins to discharge across the are, and still 
more reduces this potential difference. Owing to the inductance in 
series with the condenser, or in other words in consequence of the 
kinetic energy of the moving electrons, the condenser is not only dis- 
«The term negative resistance is a very inappropriate term. It is better to 
eall the curve for an electric are showing the relation of current through the are 
to potential difference of the electrodes or poles the characteristic curve of that 
are, following a usual nomenclature in connection with dynamos. This charac- 
teristic is a curve sloping downward when the current is taken as abscissa 
and the P, D. as ordinate. 
