Trowbridge and Duane— Velocity of Electric Waves. 105 
between them consisted of the best French plate glass obtain- 
able (k=8+ probably), and was 2™ thick. Outside the plates 
a and b, and separated from them by a hard rubber dielectric 
(k=2+ about) 1°8°" thick were the secondary plates 26 x 26™. 
The primary and secondary circuits were joined to the con- 
denser plates as indicated in the figure. The primary circuit 
lay in the horizontal plane passing through the centers of con- 
denser plates, and consisted of copper wires °34°" in diameter. 
In order to control the period of oscillation of the primary 
circuit, the portion BD, containing a spark gap with spher- 
ical terminals, was made to slide along parallel to itself. 
The distance between the straight portions AB and CD was 
40, and the lengths of AB and CD finally chosen for best 
resonance were 85°". Most of the secondary circuit lay 
in a horizontal plane 15°" above that of the primary. The 
lengths GE and HF’, however, were bent down and fastened to 
the middle points G and H of the secondary plates. The cir- 
cuit consisted of copper wire (diameter ‘215% ), and its total 
length from G through I to H was 5860™. At I was a spark 
gap with pointed terminals. With this apparatus we suc- 
ceeded in producing a very regular wave formation, as indi- 
cated by the bolometer. So many curves have been plotted 
and published, to illustrate the characteristics of electrical 
waves, that it does not seem worth while to add to the num- 
ber here. It will be sufficient to state that the ratio of the 
maximum and minimum deflections in the bolometer was about 
15:1, and that there was a node at I, and another about 40™ to 
the right of E. and F. 
As stated in our first paper, the images of the secondary 
spark were thrown on a sensitive plate by means of a rotating 
mirror. Plate III gives an enlargement (about 10 times) of 
one of the sparks taken in our final measurement. ‘The dots 
represent discharges from the negative terminals only, the 
positive discharges not being brilliant enough to affect the plate. 
The distance between successive dots was the distance on the 
plate through which the image of the spark gap moved during 
the time of a complete oscillation. Hence by determining | 
the speed of the mirror and measuring the distances from the 
mirror to the plate the time of oscillation could be calculated. 
To measure the sparks we used a sharp pointer moved at the 
end of a micrometer screw, under a magnifying glass of low 
power. The instrument was originally intended for micro- 
scopic measurements, and was very accurately constructed. 
The rotating mirror was driven by an electric motor by means 
of a current from a storage battery of extremely constant volt- 
age. ‘To give great steadiness a heavy fly-wheel was attached 
to the axis of the mirror. The speed of the mirror was 
