f it is above this capacity, cartridge fuses, costing a
little more, are required. A supply of these fuses should be kept
handy at all times.

_Governors and Voltage Regulators_

[Illustration: A centrifugal governor (Courtesy of the C. P. Bradway
Company, West Stafford, Conn.)]

The necessity for water wheel governors will vary with conditions. As
a general rule, it may be said that reaction turbines working under a
low head with a large quantity of water do not require as much
governing as the impulse wheel, working under high heads with small
quantities of water. When governing is necessary at all, it is because
the prime mover varies in speed from no load to full load. Planning
one's plant with a liberal allowance of power--two water horsepower to
one electrical horsepower is liberal--reduces the necessity of
governors to a minimum. As an instance of this, the plant described
in some detail in Chapters One and Six of this volume, runs without a
governor.

However, a surplus of water-power is not usual. Generally plants are
designed within narrow limits; and then the need of a governor becomes
immediately apparent. There are many designs of governors on the
market, the cheapest being of the centrifugal type, in which a pair of
whirling balls are connected to the water wheel gate by means of
gears, and open or close the gate as the speed lowers or rises.

Constant speed is necessary because voltage is directly dependent on
speed. If the speed falls 25 per cent, the voltage falls likewise; and
a plant with the voltage varying between such limits would be a
constant source of annoyance, as well as expense for burned-out lamps.

Since constant voltage is the result aimed at by the use of a
governor, the same result can be attained in other ways, several of
which will be explained here briefly.

_Over-Compounding_

(1) Over-compounding the dynamo. This is simple and cheap, if one buys
the right dynamo in the first instance; or if he can do the
over-compounding himself, by the method described in the concluding
paragraphs of Chapter Seven. If it is found that the speed of the
water wheel drops 25 per cent between no load and full load, a dynamo
with field coils over-compounded to this extent would give a fairly
constant regulation. If you are buying a special dynamo for direct
drive, your manufacturer can supply you with a machine that will
maintain constant voltage under the normal variations in speed of you