TRANSACTIONS OF SECTION G. 621 
A tuned station requires two capacity areas, both elevated above the earth, as 
published by me in 1897. These capacity areas are usually horizontal frames, of 
shapes devised by my friend and partner, Dr. Alexander Muirhead, who has 
found that there is a best position for the lower aérial, such that the capacity is a 
minimum. The sending efficiency is then most marked. 
If the lower aérial be too much raised, the radiating power diminishes; if it be 
lowered, the train of waves is shortened until, when it is allowed to touch the 
earth—still more if it is connected with the earth—there is hardly any train of 
waves at all, and the discharge is almost dead-beat. There is a great advantage 
in thus getting rid of earth contacts, inasmuch as variations of moisture and 
uncertainties of the soil do not enter in to confuse the problem and throw the 
tuning out, But even if the earth remained constant it would be deleterious: it 
seems by its resistance to damp out the vibrations and shortens the train of waves, 
in so far as it is allowed to exert any influence. 
Kind of Spark. 
A non-tuned station puts all the energy into a single snap, so as to produce a 
single discontinuous pulse calculated to affect every kind of station within the 
range of its power. For a tuned station this sudden snappy spark is to be avoided. 
The ideal arrangement is a spark of a sufficient number of alternations, of approxi- 
mately equal strength, no one of which is sufficient to operate, but such that the 
accumulated influence of all of them is powerful. Instead therefore of the clean 
polished metal knobs in fresh or compressed air, which are suitable for a snappy 
spark, a tuned station may employ a series of points enclosed in ionised air so as to 
maintain conduction as long as possible. The maintenance is also assisted by using 
an alternator with a curve of the right shape—not a sine curve, but a high- 
shouldered curve—so as to keep up the stimulating potential for a sufficient time. 
It is this kind of spark which, at the Lodge-Muirhead station at Elmers End, 
was photographed by Mr. Duddell in a revolving mirror, and was exhibited by him 
on Friday night. 
Method of Receiving from a Distant Tuned Station. 
The first thing is to tune up the receiver accurately. This can be done by 
a Duddell radio-micrometer, which measures the received energy satisfactorily 
although it is very small. Tuning is altered until the reading on this micrometer 
rises to a high value; then the receiving apparatus is purposely made insen- 
sitive, so that the coherer will only respond to this high value: in other 
words, to the top of the curve. The message can then be received from the desired 
station. If the receiving apparatus were left sensitive, it would be affected 
violently by the desired station, but it would pick up a number of disturbances 
from other stations. By working at the top of the curve it feels the desired 
station alone. 
Perfection of Tuning. 
In this way it was possible to receive at Hythe from Elmers End while a much 
more powerful and nearer station at Dover was making a disturbance which was 
entirely eliminated. It is easy to hear the ships in the Channel, but it is also 
easy to tune everything out and listen to the desired station alone. A 5 per cent. 
change could be made to throw this out and throw a neighbouring one in; but in 
practice it would be undesirable to try to work quite so close as that. 
With changes of that order of magnitude, however, several neighbouring 
sending stations can he made to send to several neighbouring receiving stations 
without interference. That is to say, diplex telegraphy is possible, though not 
duplex. 
Tuning at the Sending End. 
In order to economise power, it is desirable to have every part tuned. The 
aérials connected through the secondary of a peculiarly made Ruhmkorff coil 
