402 TRANSACTIONS OF SECTION A. 
3. An open or radiative circuit coupled to the condenser circuit, comprising 
an antenna or arrangement of elevated air-wires, a balancing 
capacity or counterpoise often buried in the earth, the two being 
connected through an adjustable inductance-coil. 
At the receiving station we have also three elements :— 
1. An absorbing antenna, by which the radiation from the transmitter is 
picked up, creating in it high-frequency oscillations. 
2. A condenser circuit having variable capacity and inductance coupled to 
the antenna and syntonised to it. 
3. Some form of oscillation detector, connected in series or parallel with the 
above condenser, which is affected by the oscillations, and sets in 
operation a recording or indicating device which makes a visible or 
audible signal. 
Generally speaking, at any one station the radiating and absorbing antennz 
are one and the same, and used for both purposes alternately, and each station 
has both transmitting and receiving apparatus. The functions are, however, 
not identical. What is required in the transmitting antenna is a certain height 
and also free or insulated ends. In the receiving antenna, not only height but 
surface is required, although this antenna can be laid parallel with and close to 
the earth, and earthed at both ends; but provided it is half a wave-length in 
length it will still absorb a considerable amount of energy from electric waves 
arriving in its own direction, 
In nearly all cases these oscillations are excited in the antenna by the inter- 
mittent discharge of a condenser. They are, therefore, damped or decadent 
trains of free oscillations, separated by intervals of silence. The group 
frequency, as it is called, or number of the trains of oscillations, is now usually 
five hunded to one thousand, since when using the telephone as a receiver the 
group frequency is preferably that frequency for which the telephone is most 
sensitive. Each train of oscillations may comprise thirty, fifty, or one hundred 
oscillations having the antenna frequency. The antenna is, therefore, set in 
electrical vibration, so that trains of electric currents run up and down it inter- 
mittently, say five hundred times a second, each train consisting of fifty or more 
decadent oscillations, whilst each oscillation or single current occupies a time 
between one fifty-thousandth of a second and one two-millionth of a second in 
its complete two-and-fro cycle. 
These high-frequency currents in the antenna are created by the induction of 
a nearly dead beat, or else an oscillatory discharge of a condenser. In small 
installations the condenser is a collection of Leyden jars, or, more conveniently, 
glass plates coated with thin sheet zinc or tin, the plates being immersed in a 
metal or stoneware box of oil. 
From the transmitting antenna electric radiation takes place. 
An extremely small fraction of the whole radiated energy is picked up by the © 
receiving antenna. In this latter we have currents created which are measured 
in microamperes, or at best in fractions of a milliampere. If the receiving 
antenna is properly tuned to a closed condenser circuit inductively coupled to it, 
the energy picked up by the receiving antenna accumulates in the associated 
condenser circuit. 
In this last we then have feeble currents circulating which imitate in mode of 
variation those of the distant transmitting antenna. To detect them it is now 
most usual to employ a telephone in series with some form of current rectifier, 
which is shunted across the condenser in the closed secondary receiving circuit, or 
else some form of current operated detector, such as Marconi’s magnetic detector, 
which is placed in the condenser circuit. 
If we merely connect a telephone across the condenser circuit no sound will 
be produced in it, because the frequency of the current oscillations in the 
receiving condenser circuit is too high to affect a telephone. If, however, we 
insert some device in series with the telephone which acts like a valve it will 
rectify the groups of oscillations into prolonged gushes of electricity in one 
direction, which, coming at the rate of the much lower spark frequency, say 
about five hundred or one thousand per second, create in the telephone a shrill 
sound. As these groups are interrupted at the sending station in accordance 
a 
