LXIV. E. K. SCOTT. 
Governing is generally arranged by slightly deflecting 
the nozzle so that more or less of the water is effective, 
but in one form—the Cassell—the wheel is in two halves 
which open slightly under the action of a governor and let 
some of the water through. With a low head fall the 
guides, wheel and draught tube of the turbine must be 
kept full of water and so the turbine has to be within say 
12 feet of the tail race level. 
Occasionally the tail water will, in flood time, bank up 
nearly this amount, and therefore it is common practice to 
have a vertical spindle turbine with the alternator above. 
With very high heads the water missing the bucket is 
destructive, and it is usual to provide a pool in the tail 
race for it to strike into, as was done at the Snowdon 
power house. But for this fact, the power house on a high 
head fall could be situated well above tail water level. 
Alternators.—From the electric generator point of view, 
a high head means a great saving of expense because the 
speed can be high. For example, at Barron Falls, with a 
head of 800 feet, the Pelton wheels could run at say 500 
revolutions, but at Trawool, where the average head 
would be 100 feet, the turbines would have to run at say 
100 revolutions. Assuming the same output of generator 
in each, the cost at 100 revolutions would be several times 
greater than for the higher speed. 
Rivers giving a head of only 15 feet or so are frequently 
harnessed, but it is only at great expense ; in some cases 
several turbines being coupled to one generator in order to 
get a reasonable speed for the latter. The highest pressure 
for which alternators have been wound is 15000 volts, but 
there are many running at 13,000 volts., and 10,000 to 
12,000 volts. is quite common. Where the distance of 
transmission does not exceed, say 25 miles, the alternators 
could therefore be wound for say 15,000 volts. and step up 
