unt dynamo
begins to fall after half-load is reached; and at full load, it has
fallen possibly 20 per cent. A rheostat, or resistance box on the
switchboard, makes it possible to cut out or switch in additional
resistance in the field coils, thus varying the strength of the field
coils, within a limit of say 15 per cent, to keep the voltage
constant. This, however, requires a constant attendance on the
machine. If the voltage were set right for 10 lights, the lights would
grow dim when 50 lights were turned on; and if it were adjusted for 50
lights, the voltage would be too high for only ten lights--would cause
them to "burn out."

Shunt dynamos are used for charging storage batteries, and are
satisfactory for direct service only when an attendant is constantly
at hand to regulate them.

_The Compound Dynamo_

The ideal between these two conditions would be a compromise, which
included the characteristics of both _series_ and _shunt_ effects.
That is exactly what the _compound_ dynamo effects.

A compound dynamo is a shunt dynamo with just enough series turns on
its field coils, to counteract the de-magnetizing effect of the
armature at full load. A machine can be designed to make the voltage
rise gradually, or swiftly, by combining the two systems. For country
homes, the best combination is a machine that will keep the voltage
constant from no load to full load. A so-called _flat-compounded_
machine does this. In actual practice, this voltage rises slightly at
the half-load line--only two or three volts, which will not damage the
lamps in a 110-volt circuit.

The compound dynamo is therefore self-regulating, and requires no
attention, except as to lubrication, and the incidental care given to
any piece of machinery. Any shunt dynamo can be made into a compound
dynamo, by winding a few turns of heavy insulated wire around the
shunt coils, and connecting them in "series" with the external
circuit. How many turns are necessary depends on conditions. Three or
four turns to each coil usually are sufficient for "flat compounding."
If the generating plant is a long distance from the farm house where
the light, heat, and power are to be used, the voltage drops at full
load, due to resistance of the transmission wires. To overcome this,
enough turns can be wound on top of the shunt coils to cause the
voltage to rise at the switchboard, but remain stationary at the spot
where the current is used. The usual so-called fla