1909] on Researches in Radiotelegraphy. 653 



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 

 brilliancy. At certain intervals of distance in .the space, the magnetic 

 and electric forces reverse direction in tlie 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 

 antenna, and the antenna radiates equally in all directions. If, 

 however, we employ a U-shaped antenna, as in the present ex- 

 periment, the currents being in opposite directions in the two 

 branches, then along a median line transverse to their common plane 

 their actions will neutralise each other, and the radiation will be 

 symmetrical only with respect to the plane of the antenna. In con- 

 structing an antenna intended to radiate in all directions, it is 

 necessary to connect the lower end to a large plate of metal or net- 

 work 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 potential. If we are only employing an antenna of 

 moderate capacity for short distance signalling, then an insulated 

 balancing capacity would not be of unwieldy 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 antennae the necessary insulated balancing capacity may be 

 somewhat large, and it is in every way better to put it below ground, in 

 other words, to employ an earth-plate and compensate for any slight 

 earth damping by an antenna of rather larger capacity. 



This matter is, however, only part of a much larger question, viz. 

 the function of the earth in radiotelegraphy. It is well known that 

 the nature of the earth's soil or surface between the sending and re- 



