Electric Waves along Spiral Wires. 421 
distances from the lower surface of. 1, 2, 3, &c. centimetres 
successively, and the capacity between the helix and the 
earth-wire measured in micro-microfarads. (1 micro-micro- 
farad = 10-6 of a microfarad.) 
In Table II. are given the results of the measurements. 
TasuE IT. 
Capacity of the long Helix of 5000 turns, 200 centims. in 
length, with respect to the earth-wire placed parallel to 
ih 8 and at different distances from it. 
Capacity of helix with 
respect to the earth-wire 
or the earth. 
Distance of earth-wire 
below the helix. 
2 centimetres. 51:2 5 
CO MT Os Orr CO bo 
us 
ho 
iw) 
10 39°8 4, 
| 
| 
: centimetre. 62 micro-microfarads. 
ale Harhewire removed. 36 i 
The above values are not the actual figures of observation, 
but are taken from a curve (see fig. 2, Pl. XIII.) drawn 
through the plotted observed values to do justice to them all. 
It was considered important to ascertain how far the 
closely wound helix had a capacity with respect to the earth 
different from that of a hollow metal cylinder of the same 
length and external diameter. Accordingly the helix was 
closely covered over with tinfoil and the observations 
repeated, taking the capacity of this tinfoil cylinder with 
respect to the earth-wire. These last observations are set 
out in the upper curve in fig. 2 (PI. XIII.), and they show 
that at all distances of the earth-wire there is a nearly 
constant difference ; the circumscribing tinfoil cylinder being 
about 6 per cent. greater than the helix in capacity. 
Since the tinfoil cylinder was of slightly larger diameter 
than the coil of wire, this shows that the error made in con- 
sidering the helix as equivalent to a continuous metal cylinder 
in regar rd to electrical capacity isnot large. Finally, the helix 
of wire was set upright and its capacity taken with respect 
to the earth with no earth-wire near it. This was found to be 
35 micro-microfarads, or 0°000036 microfarad. 
