FIRST EVENING DISCOURSE. 731 
determined by the friction which tends to stop the weight, or by the resistances 
and other losses in the electrical circuit. 
In practice the aérial conductor acts as a Leyden jar or condenser, It is 
charged with electricity and allowed to discharge, the current oscillating back- 
wards and forwards in the aérial during the discharge. In many installations 
Leyden jars or condensers are electrically connected to the aérial, so that the 
oscillations taking place in them are transmitted to the aérial. Any remarks, 
therefore, that I may make as to the oscillations which may be set up in con- 
densers apply equally well to the oscillations in the aérial in wireless telegraphy. 
For wireless telegraphy it is usual to charge the condenser or aérial by means 
of an induction coil or an alternator to a very high voltage, and it is allowed to 
discharge by means of a spark between the two electrodes which form the ends, 
so to speak, of a gap in the electrical circuit. As long as the pressure is low the 
spark gap is a perfect insulator; when the pressure becomes high enough the air 
between the electrodes breaks down and a spark passes the gap, becomes conduct- 
ing, and allows the condenser to discharge. The property of the spark-gap of 
passing almost instantaneously from a condition of being an insulator for elec- 
tricity to being an extremely good conductor for electricity is of the utmost value 
in the spark method of wireless telegraphy. The more perfectly the spark-gap is 
insulated before the discharge takes place, and the more perfectly it conducts after 
the discharge has taken place, the better it is for our purpose. 
If I take two electrodes sufficiently far apart in air and gradually raise the 
electrical pressure between them, the first indication that anything is going to 
happen is the formation of fine violet aigrette on the more pointed or rougher 
parts of the electrodes. This is known as the brush discharge. By gradually 
raising the pressure, this brush discharge extends further out into the air, until 
finally the air between the two electrodes becomes so strained that it breaks down 
and the real spark passes. 
The long thin spark that occurs in this case is not very suitable for wireless 
telegraphy, as its resistance is too high. Ordinary lightning flashes are good 
examples of long sparks on a very large scale. If instead of working with the 
electrodes far apart they are placed nearer together, and if the electrical pressure 
is supplied from a very powerful source, then directly the spark passes 1t forms 
a thick discharge having the appearance of a flame in which the nitrogen of the 
air is actually being burnt; a process which, it is hoped, in the future may have 
immense importance in the supply of artificial nitrates for agriculture. This 
flame-like discharge has a low electrical resistance, but has the effect, that it so 
heats or modifies the air that it is difficult to get the air to insulate again, after 
one discharge, ready for the next. 
If a large quantity of electricity is discharged through the spark-gap, and if the 
spark lasts a very short time compared with the interval between successive 
sparks, then a highly conducting spark can be obtained, as well as a good insula- 
tion between the sparking terminals when no discharge is passing. 
In order to help to bring the gap back to its insulating condition after each 
discharge, many devices are employed, such as subdividing the spark into several 
shorter sparks, cooling the electrodes, blowing air across the spark-gap, &c. 
When the condenser, or antenna, discharges through the spark-gap, oscillations are 
set up which radiate Hertzian waves. 
In practice in wireless telegraphy it is difficult to obtain a large number of 
oscillations during each discharge as corresponding with each oscillation; the 
antenna radiates energy. A large number of oscillations means, if we keep ampli- 
tude of each the same, that we are radiating a large quantity of energy. Besides 
this radiated energy, which is useful for transmitting messages, there is also energy 
wasted in heat in the spark-gap, in the conductors, in the glass or other insulation 
of the condensers. It is this useless part which we require to make as small as 
possible. 
I have lately had an opportunity to determine how many oscillations actually 
take place in a certain wireless transmission. The experiment was made by 
photographing the spark as seen in a mirror rotated at a very high speed, and it 
was found that each spark consisted of nine or ten complete oscillations. 
