710 

be due to dispersion, which required explanation. But 
there was a wide gap between this theoretical deduction 
of Maxwell and the wireless telegraphy of to-day, which 
needed many more investigations in “ pure” science 
before the bridge was complete. We at the Cavendish 
Laboratory—I was a student at the time—implicitly 
believed in its truth; but no one had received electro- 
magnetic vibrations—at any rate, to his certain know- 
ledge. The method of generating them and the means 
for measuring them were still to come. 
For the former we have to go back to a remarkable 
paper + by a very distinguished honorary member of 
this Institution, Lord Kelvin. Helmholtz® seems to 
have been the first to conceive that the discharge of 
a condenser through a wire might consist of a forward 
and backward motion of electricity between the coatings 
—a series of currents in opposite directions. Lord 
Kelvin took up the question mathematically and in- 
vestigated the phenomena. He showed that under 
certain conditions there would be oscillations of periodic 
time 27+/LC, where L is the inductance of the coil, 
and C the capacity of the condenser. These oscillations 
must, according to the theory, give rise to waves 
travelling out into space with the electro-magnetic 
velocity. Fitzgerald, at a meeting of the British 
Association, had predicted in 1883 that they might be 
produced by utilising the oscillatory discharge of a 
Leyden jar, and Sir Oliver Lodge in 1887 produced 
and detected them. For their detection the principle 
of resonance was employed. Any mechanical system 
free to vibrate has its own period of oscillation, and 
the application to it of a series of small impulses at 
intervals coincident with the free period of the system 
results in a disturbance of large amplitude. So, too, 
an electric system having capacity and inductance has 
its own period of electrical oscillation, and, if this 
coincides with the period of incoming electrical waves, 
electrical disturbances of a magnitude which can be 
detected by our apparatus are set up. _It is necessary 
that the receiver and the transmitter should be in 
tune. Lodge made use of this principle, and, by 
receiving the waves on wires adjusted to resonance 
with his Leyden jar and coil, was able to detect them. 
David Hughes, working in the early ‘eighties, had 
already detected such oscillations, but was discouraged 
from pursuing the subject. 
In 1879, in consequence of the offer of a prize by 
the Berlin Academy, the attention of Heinrich Herz, 
then a student under Helmholtz, was attracted to the 
problem of electric oscillations and their detection. He 
came to the conclusion that with the means of ob- 
servation then at his disposal ‘‘ any decided effect could 
scarcely be hoped for, but only an action lying just 
within the limits of observation.” The investigation 
was laid aside, only to be revived in 1886 by a chance 
observation of the effect of resonance in two circuits 
which happened to be in tune, and his realisation of 
the fact that herein lay the means of solution of his 
problem. His paper ‘“‘ On Very Rapid Electric Oscilla- 
tions ” appeared in Wiedemann’s Amnalen, vol. xxx. 
for 1887, and from this experiment came verification 
of Maxwell’s theory, the basis of all our knowledge of 
wireless. 
- < Phil. Mag., 1855. 
* Uber die E rhaltung der Kraft,” 1847. 
NO. 2795, VOL. I11 | 
NATURE 

[May 26, 1923, 

Fitzgerald directed the attention of English physicists 
to the work at the British Association meeting in 1888, 
and Lodge exhibited many of the effects of the waves 
at the Royal Institution in 1889. The investigations 
which led to such brilliant results were inspired by 
the desire for knowledge ; the idea of their practical 
application was entirely absent. Signalling by wireless 
waves was not foreshadowed until Crookes suggested 
it in 1892, and in 1893, the year of Sir William Anderson’s 
lecture, Lodge heard of Branly’s coherer and applied 
it to the rectification and reception of wireless waves. 
From this started the investigations of many of those 
whose names as pioneers are familiar to all. But 
another discovery in pure science was necessary to 
complete the work. 
Edison had shown in 1883 that if an nett 
electrode was inserted in an ordinary glow lamp there 
was a current of negative electricity from the filament 
to the electrode—the emission of negative electricity 
from a hot body had been observed by various experi- 
menters—and Fleming made some observations about 
this date on the Edison effect. In 1904 he applied 
them to produce a valve rectifier for high-frequency 
oscillations by connecting one pole of his receiving 
circuit to an insulated plate or cylinder within a carbon 
lamp, of which the negative electrode forms the other 
pole of the receiving circuit. When the filament is 
made incandescent, negative electricity can readily 
pass from it to the ‘insulated plate and hence into the 
receiving circuit ; the flow of positive electricity in the 
same direction is checked ; the lamp has a rectifying 
action. 
Dr. Lee de Forest improved this oscillation valve a 
little later, making it an amplifier as well as a rectifier 
by placing between the filament and the plate or 
cylinder a grid of metal wire connected to an external 
source of electromotive force, by means of which its 
potential can be varied. There is ordinarily a current 
of negative electricity passing from the filament to the 
plate—the plate current it is called—through the inter- 
stices of the grid. By varying the potential of the 
grid this current can be varied, and the conditions can 
be so adjusted that small changes in the potential of 
the grid will produce large changes in the plate current; 
the plate current is passed through the primary of a 
step-up transformer, in the secondary of which is the 
receiving telephone, and the effect is thereby made 
audible. The grid is connected to one pole of the 
circuit receiving the incoming waves, and the small 
variations of potential which they produce thus give 
rise to large variations of the plate current, and hence 
the sound is amplified. By placing a number of valves 
in series very large amplifications are possible. 
The other uses of the valve are very numerous. It 
is now employed as a transmitter for wireless work ; 
while it finds many applications as a source, or rather 
regulator, of vibrations of comparatively short period. 
The Post Office has used it as an amplifier of speech, 
while Mr. F. E. Smith has applied it as a source of 
sound in connexion with the measurement of audibility. 
The whole of this arose from Edison’s observation of 
the discharge of negative electricity from the heated 
filament. 
To quote again from the first James Forrest lecture : 
““The engineer must banish from his mind the idea 
—— 
