Ee 
WATER WHEELS AND TURBINES 489 
water on leaving the buckets is zero, and the hydraulic efficiency is unity. 
The actual efficiency of Pelton wheels is from 70 to 90 per cent. 
The Pelton wheel is suitable for situations where a comparatively small 
amount of water at a high pressure or under a great head is available. 
There are difficulties connected with the governing of Pelton wheels 
which may be here referred to. The opening through the nozzle may be 
varied by a curved stopper at the end of a screwed rod, which works in a 
nut, as shown in Fig. 786. The area through the nozzle, for a given 
energy of jet, depends, however, on the friction of the supply-pipe as well 
as on the head of water, and is determined in the manner discussed in 
Art. 409, p. 467. The central stopper, or needle as it is sometimes called, 
may be operated by a governor driven from the shaft of the wheel. A 
sudden throttling of the jet due to the action of the governor when there 
is a sudden reduction in the power required causes a sudden check on the 
flow of the water in the supply-pipe, and if this, pipe is long the result is 
a water-hammer action, which may unduly strain the pipe. This difficulty 
may be got over by providing a spring-loaded relief valve. In another 
system of governing the nozzle is at the end of a short pipe, so jointed as 
to permit of the jet being deflected so that only part of it strikes the 
buckets. A difficulty with this system of governing, however, is that a 
very considerable force is required to deflect a jet moving at a high velocity. 
The power of a Pelton wheel may be increased by having two or 
more nozzles, instead of one, directing jets in tangential directions at 
different parts of the circumference of the wheel. 
425. Girard Impulse Wheel.—One form of the Girard impulge 
wheel is shown in Fig. 789. This wheel is mounted on a horizontal 
shaft A. The water enters through a pipe B, which bends over and 
= 4 
Fra. 789. 
terminates opposite to one or more guide passages C, which direct the 
water on to the vanes D of the wheel. The quantity of water entering 
4 
