RADIOTELEGRAPHY—FLEMING. 159 
As the currents in the air wire reverse their direction the magnetic 
and electric effects in the external space also reverse, but not every- 
where at the same moment. The magnetic and electric forces are 
affections or states of the ether, and in virtue of the inertia and 
elasticity of the medium they are propagated from point to point 
with a finite velocity which is the same as that of light. We can 
then explore the field near the antenna and obtain an approximate 
idea of its nature and intensity by the use of a Neon vacuum tube, 
which glows when held in the electric field with greater or less 
brilancy. At certain intervals of distance in the space the magnetic 
and electric forces reverse direction in the same way at the same 
instant, and this distance is called a wave length. 
In the case of a straight air wire, the magnitude of the forces at 
considerable distances varies inversely as the distance from the an- 
tenna, and the antenna radiates equally in all directions. If, how- 
ever, we employ a U-shaped antenna, as in the present experiment, 
the currents being in opposite directions in the two branches, then 
along a median line transverse to their common plane their actions 
will neutralize each other, and the radiation will be symmetrical only 
with respect to the plane of the antenna. In constructing an antenna 
intended to radiate in all directions, it is necessary to connect the 
lower end to a large plate of metal or network of wires either sunk 
in the earth or placed just above the surface. In the former case, 
this plate is called an earth plate, and in the latter a balancing 
. capacity. It is necessary that this balancing capacity, if insulated, 
should be of sufficient size to take up all the electricity which rushes 
out of the antenna at each oscillation without sensible rise in poten- 
tial. If we are only employing an antenna of moderate capacity for 
short distance signaling, then an insulated balancing capacity would 
not be of unwieldly dimensions and may be constructed of a number 
of wires stretched out or laid on the ground or insulated a little way 
above it. When, however, we have to employ a very large antenna 
of great capacity for long distance work, then the provision of a 
suitable balancing capacity would involve constructive difficulties 
which are best obviated by making the earth itself the balancing 
capacity—in other words, by connecting the base of the antenna to 
an extensive network of wires or large metal plates buried in the 
ground. It has been asserted that the direct earth connection damps 
out the free oscillations in the antenna more quickly than would be 
the case if an insulated balancing capacity is employed. Although 
this may be true to a certain extent, we have to set against it the fact 
that the use of an insulated balancing capacity is out of the question 
in many cases—as on board ship, where a connection to the hull of 
the vessel is always made. Also for any but small antenne the neces- 
sary insulated balancing capacity may be somewhat large, and it is 
