166 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
force exercise a lateral pressure on each other, whilst lengthways they 
are in a state of tension, and also that lines of electric strain can not 
exist inside a conductor such as a spark. Hence when the spark 
happens, the lines which a moment ago stretched across the spark gap 
disappear. There is then an unbalanced pressure on the remaining 
lines which are thus squeezed in toward the gap and deformed, so 
that they finally extend, not from rod to earth, but from two adjacent 
places on the earth and form a semiloop. 
But, as above explained, the rod does not simply become discharged. 
Owing to the inertia of the electrons when they rush out, they more 
than discharge the rod, they overdo it and leave it positively charged. 
This, then, implies that a fresh system of lines of electric force grows 
up between the earth and the rod, and the first formed set of semi- 
loops is pushed outward. Then the process is repeated as the oscilla- 
tions of the electrons in and out of the rod die away, and in the 
space around we have a system of semi- 
loops of electric force being pushed out- 
ward in every direction as shown in the 
diagram in fig. 2.4 
There is, however, another factor in- 
volved in the process. The movement of 
the electrons into and out of the rod 
constitutes an alternating electric current 
and this is accompanied by the production 
Z of an alternating magnetic field, the di- 
rection of which is represented by a sys- 
Fie. 3.—Iines of magnetic force '©m of concentric circles with their cen- 
round an antenna during dis- ters on the antenna (see fig. ae When 
hares: the current in the rod is reversed in direc- 
tion, the field is not reversed at all parts of space instantaneously, but 
the reversal is propagated outward with the speed of light. Accord- 
ingly, the electric oscillations in the antenna create periodic variations 
of magnetic force and electric force in the space outside. At points 
near the earth’s surface some way from the rod the magnetic force is 
parallel to, and the electric force perpendicular to the surface of 
the earth or sea. Experience shows that electric wave telegraphy 
“Tn referring to lines or semiloops of electric force as moving through space, 
we do not necessarily mean to imply that each line is earmarked so that it 
preserves an individual identity. All that actually happens at any point in 
the field is a periodic oscillation or cyclical change in the electric and magnetic 
force at that point. This, however, is repeated successively from point to point, 
and we may hence speak of the line of force as moving forward just as we 
speak of a surface water wave moving forward, when in reality the only move- 
ment in the latter case is a small up and down motion of the water at each 
place, or at least a circular motion of no very great extent. 
