In this type of combination, different resistors are joined end to end. (one after the another)
Example
This is done to Increase Resistance of Circuit
In this case Total Resistance of circuit is equal to sum of Individual Resistance of the Resistors
R = R _{ 1 } + R _{ 2 } + R _{ 3 }
Example
Suppose Resistance of 2 resistors are 20 Ω and 30 Ω
Hence,R _{ 1 } = 20 Ω and R _{ 2 } = 30 Ω
Total Resistance = R _{ 1 } + R _{ 2 } = 20 + 30 = 50 Ω
IMPORTANT POINTS
When 2 Resistors are Connected in Series

Same Current flows throughout the circuit in all resistors Let it be denoted by I

Potential Difference across combination of Resistors is equal to total Potential Difference of Each Resistor
Suppose 3 Different Resistors have Potential Difference of V _{ 1 } , V _{ 2 } and V _{ 3 } Total Potential Difference V = V _{ 1 } + V _{ 2 } +V _{ 3 } Hence,Total Potential Difference is equal to voltage of the battery
 Total Resistance is equal to Individual Resistance of each Resistor Hence R = R _{ 1 } + R _{ 2 } + R _{ 3 }
How is the Resistance formula Derived  for series circuits?
Resistance of first resistor+second resistor+third resistor = Total Resistance
Hence we can say that sum of Individual Resistance of 3 resistors is equal to Total Resistance
So more than one resistors are combined together to increase resistance of the circuit
More the Resistance,less the current flows through circuit
Hence,this method is used to reduce current flow in circuit
Note  : In case of series, the total resistance is always greater than the resistance of the resistors.
The resistance produced is highest when resistors are connected in series.
Note  : An ammeter is connected in series in a circuit so that it can detect the net current flowing in a circuit.
Questions
Q1 Page 213  Draw a schematic diagram of a circuit consisting of a battery of three cells of 2V each , a 5Ω resistor, an 8 Ω resistor, and a 12 Ω resistor. And a plug key, all connected in series.
Q2 Page 213  Redraw the circuit of Question 1, putting in an ammeter to measure the current through the resistors and a voltmeter to measure the potential difference across the 12 Ω resistor. What would be the readings in the ammeter and the voltmeter?
Example 12.7 
An electric lamp, whose resistance is 20 Ω, and a conductor of 4 Ωresistance are connected to a 6 V battery (Fig. 12.9). Calculate
(a) the total resistance of the circuit,
(b) the current through the circuit, and
(c) the potential difference across the electric lamp and conductor.
Question 4  What is the maximum resistance which can be made using five
(a) 1/5 Ω
(b) 10 Ω
(c) 5 Ω
(d) 1 Ω
Question 9  A cell, a resistor, a key and ammeter are arranged as shown in the circuit diagrams below. The current recorded in the ammeter will be
(a) maximum in (i)
(b) maximum in (ii)
(c) maximum in (iii)
(d) the same in all the cases
Question 11  Why does Ammeter have low resistance? Given reason
Why does Voltmeter have high resistance?