158 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
apparatus, which, when connected to the antenna, serves to detect the 
presence in it of the very feeble oscillations which are being generated 
in the antenna by the powerful oscillations in the antenna of some 
far-distant sending station. It is usual to employ the same antenna 
at any one station both for sending and receiving, and to switch it over 
from the transmitter to the receiver according as we wish to send or 
receive messages, although methods have been described and are being 
developed for using the antenna simultaneously for both purposes. 
By way of preface let me illustrate by a few experiments the 
manner in which these electric oscillations are set up in the air wire, 
and the nature of the effects produced by them in the surrounding 
space. We have here a very long wire which, for the purpose of 
keeping it within a small compass, is coiled upon an ebonite tube. 
Two such spirals, H, and H,, are placed side by side and connected 
at the bottom through two other small coils of wire S (see fig. 1). In 
contiguity to these last two coils of wire are two others, P, which are 
in series with a condenser or battery of 
Leyden jars, C, and a spark gap. If we 
charge the condenser by an induction coil, 
I, and let it discharge across the gap, we 
produce rapidly succeeding trains of elec- 
tric oscillations in the condenser circuit, 
and these induce other currents in the open 
or helix circuit of similar kind. The re- 
sult is that electricity rushes up and down 
the spiral wires, which we may consider to 
represent two very long air wires or an- 
Fic. 1. tenne. We have therefore, alternately, 
free charges of electricity at the top ends of the wires and electric 
currents passing to and fro across the middle point. We may com- 
pare this movement of electricity in the helix to the oscillations of a 
liquid in a U-tube when it is disturbed. In the electrical case we 
have at each spark discharge 20 or 30 electrical swings or oscillations 
separated by relatively long intervals of silence, the intervals between 
two swings in the train being about one four-hundredth-thousandth 
of a second, while the interval between the groups or trains of swings 
is about one-fiftieth of a second. 
Such electrical oscillations in the wire produce two effects in 
external space, called, respectively, electric and magnetic force. In 
the case of a simple vertical air wire the magnetic force is distributed 
along concentric circular lines embracing the wire while the electric 
force is distributed along certain looped lines in the plane of the wire. 
If, however, we employ a close-wound spiral antenna, as in our 
experiment, the positions of the electric and magnetic forces are inter- 
changed as compared with those of the single vertical wire. 
