616 TRANSACTIONS OF SECTION G. 
should be equal to the resistance of the rest of the circuit. If this were true, then 
no battery could ever have an efficiency of more than 50 per cent. It was 
supposed in many quarters that this misleading rule was applicable also to the 
dynamo. The dynamo makers discovered for themselves the fallacy of this idea, 
and strove to reduce the internal resistance of the armatures of their machines to 
aminimum. Then the genius of the lamented John Hopkinson led him to apply 
to the design of the magnetic structure of the dynamo abstract principles upon 
which a rational proportioning of the iron and copper could result. A similar 
investigation was independently made by Gisbert Kapp, and between these 
accomplished engineers the foundations of dynamo design were set upon a 
scientific basis. To the perfection of the design the magnetic studies of our 
ex-President, Professor Ewing, contributed a notable part, since they furnished a 
basis for calculating out the inevitable losses of energy in armature cores by 
hysteresis and parasitic currents in the iron when subjected to recurring cycles of 
magnetisation. Able constructive engineers, Brown, Mordey, Crompton, and 
Kapp, perfected the structural development, and the dynamo within four or five 
years became, within its class, a far more highly efticient machine than any steam 
engine. And as by the principle of reversibility every dynamo is also capable of 
acting as a motor, the perfection of the dynamo implied the perfection, both 
scientific and commercial, of the motor also. The solution in the eighties of the 
problem how to make a dynamo to deliver current at a constant voltage when 
driven at a constant speed, found its counterpart in the solution by Ayrton and 
Perry of the corresponding problem how to make a motor which would run at 
constant speed when supplied with current at a constant voltage. Both solutions 
dep -nd upon the adoption of a suitable compound winding of the field magnets. 
A little later alternating currents claimed the attention of engineers; and the 
alternating current generator, or ‘alternator,’ was developed to a high degree of 
perfection. To perfect a motor for alternating currents was not so simple a 
matter. But again pure science stepped in, in the suggestion by Galileo 
Ferraris of the extremely beautiful theorem of the rotatory magnetic field, due to 
the combination of two alternating magnetic fields equal in amplitude, identical 
in frequency and in quadrature in space, but differing from each other by a 
quarter-period in phase. To develop on this principle a commercial motor required 
the ingenuity of Tesla and the engineering skill of Dobrowolsky and of Brown: 
and so the three-phase induction motor, that triumph of applied science, came 
to perfection. Ever since 189], when at the Frankfort Exhibition there was 
shown the tour de force of transmitting 100 horse-power to a distance of 
100 miles with an inclusive efficiency of 73 per cent., the commercial possibility of 
the electric transmission of power on a large scale was assured. The modern 
developments of this branch of engineering and the erection of great power- 
stations for the economic distribution of electric power generated by large steam 
plant or by water-turbines are known to all engineers. The history of the 
electric motor is probably without parallel in the lessons it affords of the 
commercial and industrial importance of science. 
But the query naturally rises: If a steam-engine is still needed to drive the 
generator that furnishes the electric current to drive the motors, where does the 
economy come in? Why not use small steam-engines, and get rid of all 
intervening electric appliances ? The answer, as every engineer knows, lies in 
the much higher efficiency of large steam-engines than of small ones. A single 
steam-engine of 1,000 horse-power will use many times less steam and coal than a 
thousand little steam-engines of 1 horse-power each, particularly if each little steam- 
engine required its own little boiler, The little electric motor may be designed, 
on the other hand, to have almost as high an efficiency as the large motor. And 
while the loss of energy due to condensation in long steam-pipes is most serious, 
the loss of energy due to transmission of electric current in mains of equal length 
is practically negligible. This is the abundant justification of the electric distri- 
bution of power from single generating centres to numerous electric motors placed 
in the positions where they are wanted to work. 
