188 SECTIONAL ADDRESSES. 
superintended their manufacture in Glasgow and their installation and 
operation. The success of the Atlantic cable was to a large extent a 
personal triumph for Lord Kelvin. Although numerous improvements 
have been made in the details of cable manufacture and in the transmitting 
and receiving apparatus, no outstanding change has been made in recent 
years in the methods of submarine telegraphy. 
Turning to another branch of electrical communication, it is no 
exaggeration to say that modern business life has been revolutionised by 
the telephone, which will shortly celebrate its jubilee, for it was in 1876 
that Graham Bell invented the magnetic telephone receiver, although others, 
notably Reis, had been working at the problem since 1861. Bell showed 
his telephone in operation at the Philadelphia Centennial Exhibition in 
1876, and Kelvin, who was one of the judges, brought one back with him 
and demonstrated it to Section A of the British Association, at its meeting 
in Glasgow in the autumn of 1876. 
A successful telephone system requires much more than efficient 
transmitters and receivers, and the great development which has taken 
place has been largely a matter of improvement in the design of the many 
elements that go to make up a telephone exchange. The modern manual 
central-battery exchange, in which one has only to lift his receiver to call 
the operator and be connected in a few seconds to any one of 10,000 other 
subscribers, is a marvel of ingenuity and construction. But this is now 
gradually being replaced by the greater marvel of the automatic system, 
in which the operator is eliminated and the subscriber automatically 
makes his own connection to the desired subscriber. Attention should be 
drawn to two outstanding inventions in the actualtransmission of telephony 
over long distances, viz., loading and repeaters. It was Oliver Heaviside 
who in 1885 proposed to improve the range by increasing the inductance 
of the line. Although this revolutionary suggestion fell on deaf ears for 
fifteen years, it ultimately proved to be one of the great inventions of 
telephony ; it is of special importance in underground and submarine 
telephone cables, the electrostatic capacity of which otherwise seriously 
limits the range. The other outstanding novelty is the introduction of 
repeaters at intermediate points in long telephone lines. These repeaters 
are specialised types of low-frequency amplifiers ; they were made commer- 
cially possible by the invention and perfection of the three-electrode 
thermionic valve. The attenuated speech currents arriving at the end of 
a section of line are amplified and thus given a new lease of life before being 
passed on to the new section. By using a large number of such repeating 
stations, telephonic communication has been established between New 
York and San Francisco. But in addition to making such long-distance 
communication possible, the use of repeaters enables medium distances 
to be bridged by relatively cheap lines of high attenuation. 
One important application of telephony which is not generally known 
is in the control of transport ; the advantage to be gained by controlling 
the whole railway traffic of a large district from a central office need only 
be mentioned to be appreciated. 
Turning now to radio telegraphy and telephony, one cannot but marvel 
at the rapidity of its development and the inroad that it has made during 
the last two or three years on the domestic life of the whole civilised world. 
The theory of Clerk Maxwell in 1864 and the laboratory experiments of 
Wr 
