ELECTRIC WAVE TELEGRAPHY—FLEMING. iN y(t 
In the next place we have the electric arc method, to which so much 
attention has lately been directed, employing a continuous current are 
with a condenser and inductance placed in series across the terminals 
of the are. As in many other cases, the seeds of this invention were 
sown in the form of discoveries by several workers. In July, 1892, 
Prof. Elihu Thomson filed a United States Patent No. 500630, in 
which he proposed a method for creating high-frequency alternating 
currents by connecting a condenser and inductance to a pair of spark 
balls and this spark gap was also connected through two other in- 
ductances with a source of continuous current supply such as a stor- 
age battery or dynamo. (See fig. 8.) An air blast or magentic field 
was employed to continually extinguish the continuous current arc 
formed. The operation of the arrangement was thought to be as 
follows: When the arc is blown out, or before it is formed, the con- 
denser is charged by the dynamo.* When the arc is reéstablished the 
condenser is discharged with oscillations. In the above specification 
nothing is said about the use of a continuous current are between 
Fig. 8.—Elihu Thomson’s method Fig. 9.—Duddell musical 
of producing oscillations. are. 
carbon poles, but Professor Thomson asserts that oscillations with 
frequency up to 50,000 could be obtained. In 1900 Mr. Duddell 
showed that if a suitable condenser and inductance was shunted across 
the poles of a continuous current are formed with solid carbons, high- 
frequency alternating currents were set up in the condenser circuit 
and the are emitted a musical sound. (See fig. 9.) 
Much discussion subsequently took place as to the causes of the 
effect and as to the highest frequency of oscillation it was possible to 
secure by this method. Duddell and others based their explanation 
of the phenomenon upon the known fact that a small decrease in the 
current through the carbon are is accompanied by an increase in the 
4 An interesting and not yery dissimilar device has recently been described 
by Mr. S. G. Brown. He employs a revolving aluminum wheel against which 
a copper spring presses lightly. The spring and wheel are connected through 
an inductance and resistance with a source of direct current supply, and also 
by a circuit consisting of Leyden jar in series with a coil of wire. When the 
wheel revolves an are is formed at the loose contact, and high-frequency oscilla- 
tions are set up in the Leyden jar circuit. (See The Electrician, November 23, 
1906, Vol. LVIII, p, 201.) 
