728 EVENING DISCOURSES. 
EVENING DISCOURSES. 
FRIDAY, AUGUST 2. 
The Are and the Spark in Radio-Telegraphy. 
By W. Duvvet., F.R.S. 
Tab discovery by Heinrich Hertz between 1887 and 1889 of experimental 
means for the production of electric waves and Branley’s discovery that the con- 
ductivity of metallic particles is affected by electric waves form the foundation on 
which, in 1896, Signor Marconi built up his system of wireless telegraphy. 
Many of the early investigators certainly had glimpses of a future system of 
being able to transmit messages without connecting wires, for as early as 1892 
Sir William Crookes predicted in the ‘Fortnightly Review’ the possibility of 
telegraphy without wires, posts, cables, or any of our costly appliances, and said, 
granting a few reasonable postulates, the whole thing comes well within the realms 
of possible fulfilment. 
Two years later Sir Oliver Lodge gave his memorable lecture on the work of 
Hertz, and carried the matter a step nearer the practical stage. 
There will not be time to dwell to-night on the early history of the art and its 
development. It will be necessary, however, to explain some of the fundamental 
properties of signalling by means of Hertzian waves in order to be able to bring 
out clearly the relative advantages and disadvantages of the two rival methods 
now in practical use for producing Hertzian waves for wireless telegraphy. 
The fundamental part of the transmitting apparatus may be said to consist of 
a long conductor, generally placed vertically, in which an alternating or oscillating 
current is set up by some suitable means. Such a conductor radiates energy in 
the form of Hertzian waves at right angles to itself into space, in very much the 
same way that an ordinary candle sends out light in all directions. This radiation, 
though it is strictly in the nature of light, is invisible to our eyes, as the frequency 
is too low. 
If we set up any other conductor approximately parallel to the first, there will 
be produced in this second conductor alternating or oscillating currents having the 
same frequency as those in the first conductor, and which can be detected by 
suitable instruments. 
The simplest, and one of the earliest methods for producing Hertzian waves 
for use in wireless telegraphy consisted in charging up by means of an induction 
coil a vertical insulated conductor, which was allowed to discharge itself to earth 
by means of a spark taking place between its lower end and another conductor 
which was connected to earth. To detect the Hertzian waves, Marconi employed 
an improved form of the Branley filings tubes, which is known as the ‘ coherer.’ 
In order to transmit messages the radiation is started and stopped so as to form 
short and long signals, or dots and dashes of the Morse code, out of which the 
whole alphabet is built up in the well-known way. 
As I have already stated, the radiation takes place round the vertical conductor 
approximately equally in all directions. Suppose that I set up my transmitting 
apparatus here in Leicester, a receiving station set up either in Nottingham, ~ 
Derby, Rugby, or Peterborough would be able to receive the message equally 
well. Should I wish to send a message from here to Nottingham at the same 
