r buildings
wires must not come closer than 7 feet to flat roofs, or one foot to a
ridge roof. Feed-wires for electric motors should be determined from
the table of safe carrying capacities, and should be of liberal size.

CHAPTER IX

THE ELECTRIC PLANT AT WORK

Direct-connected generating sets--Belt drive--The
switchboard--Governors and voltage regulators--Methods of achieving
constant pressure at all loads: Over-compounding the dynamo; A
system of resistances; (A home-made electric radiator); Regulating
voltage by means of the rheostat--Automatic devices--Putting the
plant in operation.

Dynamos may be connected to water wheels either by means of a belt, or
the armature may spin on the same shaft as the water wheel itself. The
latter is by far the more desirable way, as it eliminates the loss of
power through shafting and belting, and does away altogether
with the belts themselves as a source of trouble. An installation
with the water wheel and armature on the same shaft is called a
"direct-connected set" and is of almost universal use in large power
plants.

To be able to use such a direct-connected set, the dynamo must be
designed to develop its full voltage when run at a speed identical
with that of the water wheel. That is, if the dynamo is wound to be
run at a speed of 800 revolutions per minute, it must be driven by a
water wheel which runs at this speed and can be governed within narrow
limits. Small impulse wheels running under great heads attain high
speed, and for such wheels it is possible to obtain a suitable dynamo
at low cost. For instance, a 12-inch impulse wheel, running under a
200-foot head will develop 6-3/4 horsepower when running at a speed of
875 revolutions per minute. A dynamo for direct coupling to such a
wheel should have a rated speed within 5 per cent of 875 r.p.m.; and,
as generators of this speed are to be had from the stock of almost all
manufacturers, there would be no extra charge.

When it comes to the larger wheels, however, of the impulse type, or
to turbines operating under their usual head the question becomes a
little more difficult. In such cases, the speed of the water wheel
will vary from 150 revolutions per minute, to 400, which is slow
speed for a small dynamo. As a general rule, the higher the speed of a
dynamo, the lower the cost; because, to lower the speed for a given
voltage, it is necessary either to increase the number of