3.7 RESISTIVITY OF VARIOUS MATERIALS

The resistivities of various common materials are listed in Table 3.1. The materials are classified as conductors, semiconductors and insulators depending on their resistivities, in an increasing order of their values. Metals have low resistivities in the range of ${10}^{-8}\mathrm{\Omega }m$ to ${10}^{-6}\mathrm{\Omega }m$. At the other end are insulators like ceramic, rubber and plastics having resistivities ${10}^{18}$ times greater than metals or more. In between the two are the semiconductors. These, however, have resistivities characteristically decreasing with a rise in temperature. The resistivities of semiconductors are also affected by presence of small amount of impurities. This last feature is exploited in use of semiconductors for electronic devices.

TABLE 3.1 RESISTIVITIES OF SOME MATERIALS

Material	Resisitivity, $\rho (\mathrm{\Omega }m)$ at $0°C$	Temperature coefficient of resisitivity, $\alpha (°C{)}^{-1}$ $\frac{1}{\rho }\frac{d\rho }{dT}$ at $0°C$
Conductors
Silver	$1.6\times {10}^{-8}$	0.0041
Copper	$1.7\times {10}^{-8}$	0.0068
Aluminium	$2.7\times {10}^{-8}$	0.0043
Tungsten	$5.6\times {10}^{-8}$	0.0045
Iron	$10\times {10}^{-8}$	0.0065
Platinum	$11\times {10}^{-8}$	0.0039
Mercury	$98\times {10}^{-8}$	0.0009
Nichrome (alloy of Ni, Fe, Cr)	$\sim 100\times {10}^{-8}$	0.0004
Manganin(alloy)	$48\times {10}^{-8}$	$0.002\times {10}^{-3}$
Semiconductors
Carbon(graphite)	$3.5\times {10}^{-5}$	-0.0005
Germanium/td>	0.46	-0.05
Silicon/td>	2300	-0.07
Insulators
Pure Water	$2.5\times {10}^5$
Glass	${10}^{10}-{10}^{14}$
Hard Rubber	${10}^{13}-{10}^{16}$
NaCl	$\sim {10}^{14}$
Fused Quartz	$\sim {10}^{16}$

Commercially produced resistors for domestic use or in laboratories are of two major types: wire bound resistors and carbon resistors. Wire bound resistors are made by winding the wires of an alloy, viz., manganin, constantan, nichrome or similar ones. The choice of these materials is dictated mostly by the fact that their resistivities are relatively insensitive to temperature. These resistances are typically in the range of a fraction of an ohm to a few hundred ohms.

Resistors in the higher range are made mostly from carbon. Carbon resistors are compact, inexpensive and thus find extensive use in electronic circuits. Carbon resistors are small in size and hence their values are given using a colour code.

TABLE 3.2 RESISTOR COLOUR CODES

Colour	Number	Multiplier	Tolerance(%)
Black	0	1
Brown	1	${10}^1$
Red	2	${10}^2$
Orange	3	${10}^3$
Yellow	4	${10}^4$
Green	5	${10}^5$
Blue	6	${10}^6$
Violet	7	${10}^7$
Gray	8	${10}^8$
White	9	${10}^9$
Gold		${10}^{-1}$	5
Silver		${10}^{-2}$	10
No colour			20

The resistors have a set of co-axial coloured rings on them whose significance are listed in Table 3.2. The first two bands from the end indicate the first two significant figures of the resistance in ohms. The third band indicates the decimal multiplier (as listed in Table 3.2). The last band stands for tolerance or possible variation in percentage about the indicated values. Sometimes, this last band is absent and that indicates a tolerance of $20\mathrm{\%}$ (Fig. 3.8). For example, if the four colours are orange, blue, yellow and gold, the resistance value is $36\times {10}^4\mathrm{\Omega }$, with a tolerence value of $5\mathrm{\%}$.