Electric Waves along Spiral Wires. 419 
connexion with the condenser, stationary electric waves were 
established on the long helical conductor. The position of 
the nodes and loops of electric force can be most easily 
ascertained by the employment of a vacuum-tube filled with 
rarefied Neon. The form of the tube found most useful 
was similar to that used in spectrum analysis, viz., a couple 
of bulbs connected by a straight narrow tube, but without 
platinum electrodes. Such a tube, when held near the helix 
when the coil was in action, glows brightly with an orange 
light. If the tube is held with its axis perpendicular to 
the axis of the helix at a little distance from it, and then 
moved along parallel to it, it glows most brilliantly when 
opposite to the antinodes, but more faintly or not at all when 
opposite to the nodes. It was found that by exploring the 
field round the helix in this manner, the position of the nodes 
could be fixed within a few millimetres *. 
These experiments are preferably conducted in a partly 
darkened room. They can, however, be shown to a large 
audience in broad daylight provided suitable Neon vacuum- 
tubes are used. It is essential to enclose the active spark- 
gap in the condenser circuit in a metal box, not only to 
reduce the noise and obscure the light of the spark but also 
to prevent, as far as possible, the radiation of stray waves 
directly from the spark-balls. The induction-coil should not be 
too near, and should be screened by an interposed metal sheet. 
The experiments here described were commenced by a 
series of careful measurements of the inductances and capa- 
cities of the various portions of the apparatus. The author 
has perfected of late years methods for measuring very 
small electrical capacities and inductances f+. 
These methods permit a capacity as small as 30 or 40 
electrostatic units to be measured with very fair accuracy, 
and also an inductance of a few microhenrys. 
The condenser first employed in these experiments con- 
sisted of two leyden-jars of which the capacity had been 
measured first separately, then both in parallel, and lastly, both 
in series ; these capacities being taken at a frequency of 100. 
* Sir William Ramsay was so kind as to lend me a series of vacuum- 
tubes filled with argon, helium, neon, and other rare gases. Of these 
the neon tube gave the best results, and I am indebted to him for filling 
a couple of tubes for me with neon. These tubes glow with a bright 
orange light in the high-frequency electric field and define the nodes 
well. 
t+ See J. A. Fleming and W. C. Clinton, “On the Measurement of 
Small Capacities and Inductances,” Phil. Mag. 1903, vol. v., 6th series, 
p. 493. Also J. A. Fleming, “A Note on the Measurement of Smal] 
Capacities and Inductances,” Phil. Mag. May 1904. 
2G2 
