732 EVENING DISCOURSES. 
If all the oscillations had been of equal strength or amplitude, and if the 
receiving circuit had been similar to my pendulum in my mechanical model, then 
there would be very little to be gained by increasing the number of oscillations. 
As the oscillations die away in the spark method, two or three times this number 
would probably be required for the best effect. As a matter of experiment very 
good tuning was obtained with the wireless transmission referred to above. 
As an example of the sharpness of tuning obtainable by the spark method the 
following test carried out on the Lodge-Muirhead installation at Hythe may be 
of interest. 
The station at Hythe had to receive messages from Elmers End at a distance 
of 58 miles over land, in spite of the fact that the Admiralty station at Dover, 
only 94 miles distant, was transmitting as powerfully as it could, in order to 
produce interference, and that the regular communications were going on in the 
Channel between the shipping. It was found possible with a difference of wave- 
length of 6 per cent. to cut out the interference from the Dover station. 
In the are method of producing continuous oscillations we employ, as before, 
a condenser and self-induction ; but, instead of charging the condenser to a high 
voltage and allowing it to discharge by means of oscillations which die away, 
and then repeating the process over and over again, we actually maintain the 
condenser charging and discharging continuously without any intermission, so 
that we practically obtained a high-frequency alternating current in the aérial. 
To impress the difference on your minds, I have an incandescent lamp, which 
I switch on and oft rapidly about ten times, and then after a short time I repeat 
the same flickering of the light, and so on. The flickering of the light corresponds 
with the oscillations in the ordinary spark method, and the time spaces between 
the flickers represent the times during which the condenser or antenna is being 
charged ready to produce a fresh series of oscillations. In practice we may have 
as many as, say, a couple of hundred discharges of the condenser a second, and 
during each discharge we may get, say, ten complete oscillations, each oscillation 
lasting one millionth of a second, if the wave-length is 300 metres; thus the 
total time that the condenser is discharging is only one one-hundred-thousandth 
of a second, or one five-hundredth part of the interval of time between two 
successive discharges, My lamp here flickers about five times per second, and 
makes ten flickers before it goes out; the total time that it is flickering is two 
seconds, and the time before it should start to flicker again to correspond with 
the practical wireless case is therefore 1,000 seconds, or rather over a quarter of 
an hour. If now I represent continuous oscillations, such as are obtained by the 
are method with this lamp, I shall simply keep the lamp flickering continuously, 
and there will be no intervals whatever. 
The are method of producing continuous oscillations is founded on my musical 
arc. In order to explain this I must demonstrate some of the properties of the 
direct-current arc. If I vary the current flowing through the are very slowly and 
note the potential difference corresponding with each value of the current, keeping 
everything else constant, I obtain a curve generally spoken of as the characteristic 
of the are. These curves under different conditions have been very thoroughly 
investigated by Mrs. Ayrton. 
With the carbon arc between electrodes in air the voltage decreases very rapidly 
when the current is gradually increased, starting from very low values. As the 
current becomes larger the rate of decrease of the voltage becomes less and less 
until it is, comparatively speaking, quite small, with a current of 10 or 12 amperes. 
With the are between metal electrodes similar results are obtained, except that 
the discontinuity in the curves, called the hissing point by Mrs. Ayrton, takes 
place at very small currents, generally well below an ampere. 
With arcs burning in hydrogen, Mr. Upson has found that the curves are 
generally much steeper for the larger values of the current than for the cor- 
responding ares burning in air, This point is of great importance as explaining 
the value of the hydrogenic atmosphere used by Poulsen and referred to later. 
In general, I may therefore say for the above arcs that increase in current 
through the are is accompanied by decrease of the potential difference between its 
oe ee 
