168 
FESSENDEN. 
so that abnormally large rushes of current react on the fields, 
and so reduce the power to a maximum of not more than 
about twice the amount normally given out by the machine. 
Second. Since the permissible peripheral speed as limited 
by mechanical conditions is about the same for large and 
small machines— i. e. y about one and one-half miles per 
minute—with large generators we have a small number of 
revolutions per minute, and hence, to get the necessary fre¬ 
quency of alternations, a large number of poles. Since with 
180 poles an angular displacement of two degrees would en¬ 
tirely reverse the phase of alternators working parallel with 
one another, it follows that the permissible variation in an¬ 
gular displacement is very small, being measured by hun¬ 
dredths of 1 per cent. This necessary constancy of angular 
velocity has been secured by the use of heavy masses for fly¬ 
wheels, in most cases the generator field or armature being 
so built as to have high angular momentum. 
Third. The use of rotary transformers for distribution 
work in recent stations and the great variations in load 
thrown on them give rise to complex reactions tending to 
distort the fields of the generators, tending to shift the phase 
and change the wave form. These latter again react on the 
transformers with very serious results. This difficulty has 
been overcome by careful design, in which the utmost scien¬ 
tific ability has been required and has been shown, and by 
the employment of new devices, mostly in the shape of sub¬ 
sidiary conducting circuits, which effect a dampening of 
these disturbances. 
The most notable event of the year has been the appear¬ 
ance of Parson’s steam turbine in the field of large gener¬ 
ators. Turbines as large as 1,500 K. W. have been built, 
and one to give 10,000 has been designed and its construc¬ 
tion is being considered. 
With the great advantages which this type of prime motor 
possesses— i. e., small size and weight, high mechanical effi¬ 
ciency, adaptability to the use of superheat to practically 
any degree owing to absence of necessity for any internal lu- 
