APRIL 2, 1914] 
NATURE 
III 
as it travels along the cable, and its rapid attenua- 
tion or diminution in amplitude. 
When speech is uttered to the mouthpiece of 
the transmitter, the current flowing into the line 
is modulated in a complicated manner, but this 
variation in virtue of Fourier’s theorem can be 
analysed into the sum of a number of currents 
of simple harmonic or sine wave form placed in 
certain relative phases and having certain ampli- 
tudes. The velocity W, with which any simple 
harmonic current travels along a cable having a 
resistance R ohms, an inductance of L henrys, a 
capacity of C farads, and a dielectric leakance of 
S mhos per unit of length, provided that the 
quantities R/pL and S/pC are small compared 
with unity, can easily be shown to be expressed 
by the formula: 
I I 
VGE S \2 
/ 14 oe ae) 
where p=2m7 times the frequency. 
Accordingly, the greater the frequency, the 
greater will be the wave velocity. In other words, 
short waves travel faster than long. The short 
waves, having also the least energy, attenuate 
most rapidly. 
The result of this is that in the case of tele- 
phony along wires the different harmonic constitu- 
ents of the current get out of step, and degrade 
unequally. Hence the wave form, and _ the 
quality of the received sound, is altered after 
the wave has travelled a certain distance along 
the wire or cable. The result-is to diminish the 
loudness and reduce the clearness of the speech 
heard. Therefore, beyond a certain distance the 
articulate sound becomes unintelligible. 
On the other hand, it is well known that the 
velocity of electromagnetic waves through space 
is independent of the wave length, and there is, 
therefore, in this respect a marked difference 
between the transmission of electromagnetic 
waves guided along wires, and free electromag- 
netic waves diverging through space. As soon 
as the telephonic or aural method of receiving 
Morse signals in wireless telegraphy was substi- 
tuted for the method of employing some form of 
coherer as a relay to actuate a Morse inker print- 
ing them in dot and dash on paper tape, the sug- 
gestion was made that it might be possible to 
transmit articulate speech by space electromag- 
netic waves, and not merely Morse signals com- 
posed of long and short sounds; and hence to 
conduct a wireless or lineless telephony. 
It was at once recognised that before this could 
be done it would be necessary to provide a gene- 
rator of electromagnetic waves giving truly con- 
tinuous waves, and not merely intermittent groups 
or trains of rapidly decadent waves. The dis- 
covery of the power of the continuous current arc 
between carbon electrodes to create high-frequency 
oscillations in a condenser circuit connected be- 
tween the carbons, held out hopes of making such 
a generator. It was not, however, until Poulsen 
discovered the peculiar properties of an electric arc 
formed in hydrogen or coal gas to enable very 
WO. 2316,) VOL. 93/| 
W= 
bole > ae 
high-frequency oscillations to be so generated that 
progress began to be made. The Poulsen arc- 
generator consists, as is well known, of a direct- 
current arc formed with an electromotive force 
of about 500 volts between a carbon and a copper 
electrode in an atmosphere of hydrogen or coal 
gas. A strong transverse magnetic field is also 
applied to the arc. An inductive circuit, con- 
sisting of a coil of wire having in series with it 
a capacity, the capacity and inductance being so 
adjusted that the natural frequency of this oscil- 
lation circuit is not less than about 50,000, or 
preferably much higher, even up to 250,000, is 
then connected between the carbon and copper 
electrodes. Powerful continuous electric oscilla- 
tions are then set up in this condenser circuit. 
These oscillations can be made to set up similar 
oscillations in an open radiative circuit or antenna, 
which is inductively connected with the condenser 
circuit, as in the case of an ordinary wireless 
telegraph transmitter. 
In this manner continuous or uninterrupted elec- 
tric waves of a wave length which is anything 
between 1 and 4 or 5 miles, can be radiated 
from the aerial wire. The reason the hydrogen 
or coal gas is effective in enabling the arc to 
create more powerful high-frequency oscillations 
is that it increases the steepness of the character- 
istic or volt-ampere curve of the arc, and hence 
increases the energy which is conveyed to the 
condenser at each oscillation. 
To transmit speech we have then to modify 
the amplitude of these radiated continuous electric 
waves sent oat from the sending station antenna 
in accordance with the wave form of the speaking 
voice. This is done usually by some form of 
carbon microphone inserted in the base of the 
sending station antenna. When the diaphragm 
of the microphone is acted upon by the voice, the 
carbon granules in it are more or less compressed, 
and the electrical resistance thereby altered. If 
then the high-frequency current in the antenna is 
made to pass through this microphone, the ampli- 
tude of the radiated continuous waves will be 
varied by making speech to the microphone, in 
such a way as to create waves upon waves, or to 
alter the wave amplitude in accordance with the 
wave form of the speaking voice. 
At the receiving end all the arrangements are 
identical with those required in wireless tele- 
graphy when using a telephone and rectifier of 
some kind to receive audible Morse signals. The 
receiving antenna is coupled to a closed condenser 
circuit, and to the terminals of this condenser is 
attached a Bell receiving telephone in circuit with 
some oscillation rectifier, such as a crystal or 
constant detector, viz., carborundum, perikon, 
or zincite-chalcopyrite, or an ionised gas rectifier 
such as a Fleming glow-lamp valve. The tele- 
phone is then not affected by the rectified con- 
tinuous oscillations per se, but it is affected by 
the variations in their amplitude produced by the 
microphone in the transmitter circuits. 
Audible and intelligible speech can thus be re- 
produced at the receiving end. The limitations 
| that present themselves in this transmission are 
