total current is known. If the total current is not known, other means

must be used for finding the total resistance.

The simplest case is

that of several equal resistors connected in parallel.

To solve this

problem, divide the resistance of one piece of equipment by the number

of pieces connected in parallel. If two 10-ohm resistors are connected

in parallel, the total resistance offered by the two is 10/2, or 5

ohms.

If three 12-ohm resistors are connected in parallel, the total

resistance offered by the three resistors is 12/3, or 4 ohms. In other

words, the total resistance of equal resistances connected in parallel

is equal to one resistance divided by the number connected.

(2) Unfortunately, the different pieces of equipment used in electrical

circuits do not have the same resistances. Therefore, they do not all

draw the same amount of current when different pieces of equipment are

connected across a battery.

For such cases, another method has been

found for the calculation of the total resistance. Refer to figure 6

which shows two resistors of unequal value placed in parallel across a

battery.

Figure 6.

Two unequal resistors connected in parallel.

(3) The most commonly used method of determining the resistance of a

parallel circuit is known as the product-over-the-sum method as is

shown below.

6.

SERIES-PARALLEL CIRCUITS

Series-parallel circuits consist of groups of parallel resistors in series

with other resistors.

a. Solving Series-Parallel Circuits. Series-parallel circuits may be solved

by application of the rules already given for simple series and simple parallel

circuits.

To do this, the series-parallel circuit is reduced to an equivalent,

simplified circuit.

Each group of parallel resistors is first replaced by its

equivalent single resistance, and then the entire circuit is treated as a series

circuit. Refer to figure 7.