168 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
with the antenna. Every such circuit containing capacity and induct- 
ance has a natural period of electric oscillation, resembling in this 
respect the time of swing of a mechanical system composed of a heavy 
body suspended by an elastic spring.*. In my cymometer the condenser 
part consists of one to four sliding tubes, each consisting of a pair of 
brass tubes, separated by an ebonite tube. The outer tubes can slide 
off the inner ones and so vary the capacity. The inductance consists 
of a long spiral of copper wire, and the circuit is completed by a 
thick copper bar. Matters are so arranged that when the outer tubes 
are drawn off the inner tubes so as to vary the electrical capacity, the 
effective amount of the spiral included in the circuit is simultaneously 
varied in exactly the same proportion. To determine when the time 
period of the cymometer circuit is in agreement with that of the 
antenna, I use a neon vacuum tube. Some three years ago I found 
that such a tube was extremely sensitive to a high frequency electric 
field, being caused to glow brilliantly when subjected to its action. 
You are already familiar with the beautiful method discovered by 
Sir James Dewar for obtaining neon from atmospheric air by the use 
of charcoal at very low temperatures, and tubes filled with rarefied 
neon prepared by his process are able, as I have shown, to serve im- 
portant purposes in connection with wireless telegraphy. 
In the cymometer a neon tube is connected to the opposite coatings 
of the condenser. If then the cymometer bar is placed near to the 
lower part of a transmitting antenna, and we slide along the outer 
condenser tube, thus varying the capacity and inductance of the in- 
strument, provided it has a suitable range, a position will be found in 
which the neon tube glows brightly. The cymometer is equipped 
with a scale which shows for every position of its handle the corre- 
sponding frequency or time period, and the related wave length. 
Hence the simplest operation, which a child can perform, serves to de- 
termine in one instant the frequency of the oscillations in the antenna 
and the wave-length of the radiated waves. I have devised instru- 
ments of this type covering the whole range of wave-length measure- 
ment from 50 to 100 feet up to 20,000 feet or more. An instrument 
of the same kind, but with a more sensitive oscillation detector than 
a neon tube, can be used to measure the wave-length of waves being 
received on the antenna. The cymometer has other uses besides wave- 
length measurement. One of these is to draw a resonance curve and 
thence reduce the rate of decay of the oscillations in a train and their 
number. In a train of oscillations the time period occupied by each 
oscillation, whether of current or potential, is the same, but the am- 
“Tf the capacity C is reckoned as usual in microfarads, and the inductance L 
in centimeters, then the time period T of the oscillation is given by the formula 
T=,/CL/5033000. 
