r
wheel.

_A System of Resistances_

(2) Constant load systems. This system provides that the dynamo shall
be delivering a fixed amount of current at all times, under which
circumstances the water wheel would not require regulation, as the
demands on it would not vary from minute to minute or hour to hour.

This system is very simply arranged. It consists of having a set of
"resistances" to throw into the circuit, in proportion to the amount
of current used.

Let us say, as an example, that a 50-ampere generator is used at a
pressure of 110 volts; and that it is desirable to work this plant at
80 per cent load, or 40 amperes current draft. When all the lights or
appliances were in use, there would be no outside "resistance" in the
circuit. When none of the lights or appliances were in use (as would
be the case for many hours during the day) it would be necessary to
consume this amount of current in some other way--to _waste it_. A
resistance permitting 40 amperes of current to flow, would be
necessary. Of what size should this resistance be?

The answer is had by applying Ohm's Law, explained in Chapter Five.
The Law in this case, would be read R = E/C. Therefore, in this case R
= 110/40 = 2-3/4 ohms resistance, would be required, switched across
the mains, to keep the dynamo delivering its normal load.

The cheapest form of this resistance would be iron wire. In place of
iron wire, German silver wire could be used. German silver wire is to
be had cheaply, and is manufactured in two grades, 18% and 30%, with a
resistance respectively 18 and 30 times that of copper for the same
gauge. Nichrome wire has a resistance 60 times that of copper; and
manganin wire has a resistance 65 times that of copper, of the same
gauge.

First figure the number of feet of copper wire suitable for the
purpose. Allowing 500 circular mills for each ampere, the gauge of the
wire should be 40 x 500 = 20,000 circular mills, or approximately No.
7 B. & S. gauge. How many feet of No. 7 copper wire would give a
resistance of 2-3/4 ohms? Referring to the copper wire table, we find
that it requires 2006.2 of No. 7 wire to make one ohm. Then 2-3/4 ohms
would require 5,517 feet.

Since 30 per cent German silver wire is approximately 30 times the
resistance of copper, a No. 7 German silver wire, for this purpose,
would be 1/30 the length of the copper wire, or 186 feet. If nichrome
wire were used, it would be 1/60th the length of copper for