436 Propagation of Electric Waves along Spiral Wires. 
/| x e e / 
as the one P, on the oscillating circuit, and these plates are 
placed parallel to each other at a distance of a few centi- 
metres more or less. The helix is provided with a curved 
metal saddle Sl which slides on it, being made to fit closely by 
a few layers of interposed tinfoil, and this saddle is connected 
by a flexible wire with the earth. Three vacuum-tubes V 
chbuld be provided, which may be conveniently attached to a 
sort of lazy-tongs arrangement whereby they can be spaced 
out at equal distances. The process of measuring the fre- 
quency of the oscillating circuit consists in sliding the earthed 
saddle along the helix until a position is found in which there 
is a node of potential N halfway between the saddle and the 
end plate, and an antinode Anat the middle of each half of the 
section of the helix on either side of the node. When this 
position is found, the length of the helix interposed between the 
end plate P, and the saddle SI is equal to one wave-length on 
the helix. If the velocity of electric waves along this helix 
has previously been determined as described in this paper, 
the quotient of this measured velocity by the observed length 
of the wave on the helix gives the frequency of the oscilla- 
tion. Having obtained the frequency of the oscillating 
circuit, we divide this frequency reckoned in fractions of a 
second into 1000 millions, a number representing very nearly 
the velocity of Hertzian waves in free space in feet per second, 
and the quotient is the length of the aerial zether wave in feet. 
The velocity of free ether waves in space is sufficiently 
near to 10° feet per second to make the above rule quite 
accurate enough for practical purposes. 
On comparing this wave-length with the length of the aerial 
we can decide at once whether the wave being sent out is the 
fundamental or a higher harmonic of the radiating antenna. 
In another place the writer has suggested the term kuma- 
scope (from xupa, a wave) as the name for appliances of all 
kinds used for detecting electric waves. He would now 
suggest the term kummeter (analogous to ammeter) as the 
name for the above described instrument for measuring the 
length of electric waves. An instrument of this kind is of 
oreat practical use in connexion with Hertzian wave-tele- 
oraphy, as it enables us to compare the wave-length of the 
radiation used with its telegraphic effectiveness. Thus a 
wave 300 feet in length travels well over sea-surface, but it 
will not go well across towns and Jand, whereas a wave of 
five times that length travels well across anid districts thickly 
covered with hones or trees or irregular elevations. As an 
example of such a measurement the follow ing figures may be 
given, Hmploying the above described ebonite-core helix on 
