Colour Code for Carbon Resistors

Carbon resistors, used constantly in the physics laboratory, are built from a ceramic core coated with a thin layer of crystalline carbon; they are prized for being inexpensive, electrically stable and physically compact. Rather than printing the resistance value as text (which would be difficult to read on such a small component), the value is encoded using coloured rings painted around the resistor's body, following the rules of Table 2.2.

Three coloured rings encode the numerical value: the first ring gives the first significant digit, the second ring gives the second significant digit, and the third ring gives the power-of-ten (decimal) multiplier that these two digits are multiplied by. An optional fourth ring, coloured either silver or gold, gives the resistor's tolerance -- gold means $\pm5\%$ and silver means $\pm10\%$; if there is no fourth ring at all, the tolerance is understood to be $\pm20\%$. For example, a resistor with rings green (digit 5), blue (digit 6), orange (multiplier $10^3$) and gold (tolerance 5%) has value $56\times10^3\ \Omega = 56\ \text{k}\Omega$, with a tolerance of $\pm5\%$ (Figure 2.12). A practical convention for reading any real resistor correctly …