Conversion from Old Imperial Ohms to Metric Units

Although Imperial Ohms are very rarely used nowadays, it is still important to be aware of their existence, especially when working from old texts. Although in lower ranges values are almost identical to their metric counterparts, at higher values they vary drastically. This can occasionally cause serious implications even at lower ranges, especially when working with logarithmic or exponential functions.

Imperial ohms were the original unit of resistivity, named to honour the work of German physicist Georg Simon Ohm (1789-1854). His most important discovery (of 1826) now bears his name: Ohm's law describes the relationship between voltage current, and resistance. Ohm's importance was not recognised through most of his lifetime, but in 1852 he became professor of physics at the University of Munich.

The units used to measure resistance; Ohms (Ω); are also named after Ohm. It would be almost yet another hundred years before the Ohm unit would be correctly standardised.

Ohm had to improvise many clever and unusual ways to measure the resistivity of materials. As such, many of them did not give accurate results. One number in particular would become very significant - the resistance that Ohm had defined to be ∞ was actually 73.2MΩ. Ohm had found any materials above this resistance to be immeasurable, and thought that this must be the threshold above which no current can pass.

It has been widely known since the 1920s that this had been inaccurate. All materials have resistance, and as such it is always theoretically measurable, even if not in practice. At that time scientists rarely needed to consider resistances above even 1MΩ, so it rarely caused any trouble; it was simpler for everyone in the world to use the same well established standards. It was not until the late 1950s, when many advances were being made with semiconductors, that working with these units began to trouble many physicists and engineers. Something had to be done. In 1962 the world's scientific community came together and redefined the Ohm, so that an infinite resistance really would be infinite. This new definition of the Ohm would now be affectionately know as the metric Ohm; the old definition is now referred to as the Imperial Ohm, although it is rarely ever used. Resistances below roughly 2MΩ are roughly the same by both standards. Above this level, the differences between the two become much greater. A conversion table for the two standards is below:

It can be seen even at a glance that the old system would be unworkable with the far larger resistances regularly encountered today.

Calum Matheson 2004