104 Trowbridge and Duane— Velocity of Electric Waves. 
Art. XI.—On the Velocity of Electric Waves; by JOHN 
TROWBRIDGE and WILLIAM DuANE. With Plate III. 
In the April number of this Journal (vol. xlix, 297), we 
published a preliminary paper on the determination of the 
velocity of electric waves. The wave length was determined 
by means of a bolometer and the period of vibration by photo- 
graphing the spark in the secondary circuit. Since the waves 
in the secondary were not well formed when the spark gap 
was inserted, it seemed desirable to find an arrangement that 
would produce simultaneously a good waveand a spark that could 
be photographed. A number of condensers with plates of dif- 
ferent sizes and shapes, and different substances for the dielectric 
were tried, and the apparatus to be described was finally adopted. 
The difficulties to be overcome were these. Too strong a reac- 
tion between the primary and secondary condensers could not 
be employed, because the increase in the damping of the pri- 
mary due to the large amount of energy drawn off by the 
secondary, made good resonance impossible. The amount of 
energy in the primary at full charge must be much greater 
than that in the secondary. On the other hand, the capacity of 
the primary condenser must not be too great, for the self-induc- 
tion of the primary circuit would have to be proportionately 
small, and this, too, means an increase in the damping. The 
secondary condenser, too, must have a capacity less than a cer- 
tain magnitude in order that the node may fall on the cireuit 
and not in the condenser plate.. These points seem to indicate 
that small condensers are preferable to large ones, but a decrease 
in the size of the plates means a decrease in the light of the 
secondary sparks, and the sparks at the best can barely be pho- 
tographed. Practically, therefore, our choice was much limited, 
and the particular arrangement to give the best results had to 
be selected by experiment, after a long series of trials. The 
arrangement and dimensions of the apparatus finally adopted 
were as follows: 
Two metallic plates a and 0 (fig. 1), 30x30, placed in ver- 
tical planes, formed the primary condenser. The dielectric 
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