What is S.W.G.?
S.W.G. is an abbreviation for Standard Wire Gauge. Also known as British Standard Gauge. It is used as a measure of thickness in guitar strings and electrical wires. Use of S.W.G. has fallen greatly in popularity. Cross sectional area in square millimetres is now a more popular size measurement.
Comparison of wire thickness using S.W.G.
s.w.g. 16 (diameter 1.63 mm) > s.w.g. 26 (diameter 0.46 mm) > s.w.g. 36 (diameter 0.19 mm).
So, the bigger the value of s.w.g., the thinner the wire.
Factors affecting the resistance R of a given wire
(1) Length L: Bigger length, bigger resistance.
(2) Cross-sectional area A: Bigger area, less resistance.
(3) Resistivity ρ: Bigger resistivity, bigger resistance.
Resistivity is a measure of how strongly a material opposes the flow of electric current. S.I. unit is Ω-metre. Different materials have different resistivities.
Examples: silver 1.6 x 10-8 Ω-metre / copper 1.7 x 10-8 Ω-metre / gold 2.4 x 10-8 Ω-metre / eureka 49 x 10-8 Ω-metre.
Do
not confuse resistance R with resistivity ρ. They are different.
Calculating the resistance R of a given wire
Formula : R = ρL/A
where ρ is the resistivity of the material of wire, L is the length of wire and A is the cross-sectional area of wire.
(a) What is the resistance of 1 metre length of eureka wire of s.w.g. 26?
Given ρ = 49 x 10
-8 Ω-m, and L = 1 m, and A = πr
2 = 3.14 x (0.46/2) x (0.46/2) = 0.166 mm
2 = 1.66 x 10
-7 m
2.
Therefore, R = (49 x 10
-8 x 1)/ 1.66 x 10
-7 = 2.95 Ω.
(b) What is the resistance of 1 metre length of copper wire of s.w.g. 26?
Given ρ = 1.7 x 10
-8 = 0.000000017 Ω-m, and L = 1 m, and A = πr
2 = 3.14 x (0.46/2) x (0.46/2) = 0.166 mm
2 = 1.66 x 10
-7 m
2.
Therefore, R = (1.7 x 10
-8 x 1)/ 1.66 x 10
-7 = 0.10 Ω.
For the same length of wire of equal thickness, compare the resistance of copper and eureka. It is easy to see why copper is used as connecting wires in an electrical circuit.
An instrument to measure S.W.G.
Below is a disc instrument to measure S.W.G. Simply slip the wire to fit one of the holes. Read the
corresponding number which gives the value of its S.W.G.
