c. Voltage.

A certain amount of force is required to move the electrons

through each of the resistors. By using a voltmeter, we can measure the force. In

the circuit shown in figure 4, voltmeters are placed across each of the resistors

and each meter indicates the voltage required to force the current through that

particular resistor.

(1) The following voltages are present in this series circuit: 12 volts for

each of the 12-ohm resistors, 34 volts for the 34-ohm resistor, and 50

volts for the 50-ohm resistor.

When we add these voltages, we find

they total 120 volts, the value of the source voltage.

(2) This gives rise to the basic rule: The total of all the voltages

developed across the several resistances in a series circuit is always

equal to the applied voltage.

(3) The voltage across each resistance is called the voltage drop to

signify that the voltage around the circuit gradually decreases as the

current travels around the circuit from a given starting point. After

the current has made a complete loop of the circuit, the total voltage

drop equals the applied voltage.

(4) The voltage occurs only so long as current flows. If current ceases,

there is no longer a voltage drop. For example, assume that a resistor

burns out. When that happens, current flow ceases, at which time the

full applied voltage will appear across the terminals of the burned out

resistor.

d. Laws.

The three important laws relating to series circuits are:

(1) The total resistance is equal to the sum of the individual resistances.

(2) The same current flows in each part of the circuit.

(3) The sum of the voltages across the individual resistors is equal to the

applied voltage.

5.

PARALLEL CIRCUITS

A parallel circuit is one in which one terminal of each element is connected

to a common point to form one terminal of the system, and the other terminal of

each element is connected to a second common point to form the other terminal of

the system.

The schematic diagram of a parallel circuit is shown in figure 5.

This circuit shows three resistors connected in parallel between points a and b.

a. Voltage.

In figure 5, the same voltage that is applied to R1 is also

applied to R2 and to R3.

This is true because the corresponding points of each

resistor are connected to the same points, a and b, and the same difference of

potential must exist between points a and b for all three resistances.

b. Current.

If an additional path through which the current may flow is

provided in a circuit, the total current in the circuit must be the original