G.—ENGINEERING. 188 
The realisation of the idea of supplying electric current from a power 
station for lighting houses in the neighbourhood owed much to the energy 
and business ability of Mr. Edison. He exhibited his first ‘ Jumbo’ 
steam-driven dynamo in 1881, and installed two sets at Holborn Viaduct 
in the following year to supply current to neighbouring premises. The 
output of these sets was about 90 kilowatts at 110 volts, which was so 
much larger than anything previously constructed that the name ‘Jumbo’ 
was applied to these sets. About 1890 the multipolar type began to replace 
the bipolar type for the larger sizes. The size of the single units employed 
in power stations gradually increased with the increasing demand, and 
by 1895 dynamos of 1,500 kilowatts had been installed. 
As in all other types of machinery, the output obtainable from a given 
size has been gradually increased by improvements in the electrical, 
magnetic, and mechanical properties of the materials employed, and by 
improving the design so as to remove ever further the limits imposed by 
heating, sparking, voltage drop, etc. The freedom from trouble of the 
enormous number of electric trams and trains, to take only one class, is 
a testimonial to the reliability of the modern direct-current motor. 
The alternator has had a more varied development than the dynamo, 
mainly because of the absence of the commutator. The necessity of keeping 
the brush gear stationary and accessible and therefore allowing the com- 
mutator and armature to rotate led to an early standardisation of type 
in the D.C. machine. In the alternator there was no such limitation, 
and whether the field system should be inside or outside the armature 
and which of the two should rotate were largely matters of choice. There 
are great advantages in having the armature, which usually carries a high- 
voltage winding, stationary, and the usual practice has been for the field 
system to rotate within the armature. The most striking and best-known 
exception is the umbrella type of alternator installed in the first Niagara 
power station, in which the field system rotates outside the armature. 
The design of alternators has been controlled to a large extent by the 
development of the prime mover. On the Continent of Europe the slow- 
speed horizontal steam-engine led to the construction of alternators of 
enormous diameter in order to get the necessary peripheral speed, the axial 
length being consequently reduced to a few inches. In several cases these 
machines reached such a height that the travelling cranes in the erecting 
shops were useless, and special tackle had to be erected in order to assemble 
the machines. In England the high-speed marine-type engine was generally 
preferred, and consequently the alternators had a smaller number of 
poles and a smaller diameter. All this has now been modified by the 
development of the steam turbine. 
Ferranti was apparently the first to suggest that the power station 
should be outside the city, at a point convenient for fuel and water supply, 
and that the power should be transmitted into the city by high-voltage 
alternating currents. In 1890 he built the Deptford Station for the London 
Electric Supply Company, and installed 1,000-kilowatt 10,000-volt alter- 
nators. This was the pioneer high-voltage underground cable transmission, 
and much was learnt concerning the peculiarities of alternating currents 
when transmitted over cables of considerable capacity. The following year, 
1891, saw the first long-distance transmission by means of overhead 
conductors in connection with the electrical exhibition at Frankfort-on- 
