680 Professor J. A. Flemmg [May 24, 



that they finally extend, not from rod to earth, but from two adjacent 

 places on the earth and form a semi-loop. 



But as above explahied 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 outwards. Then the process is repeated as the 

 oscillations 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 outwards in every direction as shown in the diagram in 

 Fig. 2.* 



There is, however, anotlier factor involved 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 

 of an alternating magnetic field, the direction of which is represented 

 by a system of concentric circles with their centres on the antenna 

 (see Fig. 3). When the current in the rod is reversed in direction, 



,--^^. z"*^. 



Fig. 2. — Diagrammatic Repeesentation of the Detachment of Semi- 

 Loops OF Electric Strain from a Simple Marconi Antenna. 



the field is not reversed at all parts of space instantaneously, but the 

 reversal is propagated outwards 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 

 over any large distances cannot be conducted unless the antenna is 

 so placed that the electric force is perpendicular to the surface of the 

 earth or sea. 



* In referring to lines or semi-loops of electric force as moving through 

 space, we do not necessarily mean to imply that each line is ear-marked s;o 

 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 movement 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. 



