Our Origins - Thomson Bridge

Our namesake William Thomson

Our Company is named in honour of a great thinker, inventor and educator, William Thomson, also known as Lord Kelvin.

Thomson was a mathematical physicist and engineer. Born in Belfast in 1824, he helped develop the second law of thermodynamics and he invented the absolute temperature scale named after him.

We are inspired by his career, his wiry intellect and problem-solving abilities and his significant inventions such as Thomson’s Bridge.

Today, the Thomson Bridge team strives to deliver education solutions that contribute to the quality of the work performed and the safety of those who undertake these activities.

An illustrious career

During Thomson’s long and esteemed career, he was chief consultant for the laying of the first Atlantic cable (1857–58). His work in electricity and magnetism led to James Clerk Maxwell's theory of electromagnetism. He also contributed to the determination of the age of the Earth and the study of hydrodynamics.

Thomson was also an electric telegraph engineer and inventor, which propelled him into the public eye and ensured his wealth and fame. He had extensive maritime interests and was most noted for his work on the mariner's compass, which had previously been limited in reliability.

A Kelvin or Thomson bridge is a measuring instrument used to measure unknown electrical resistors below 1 ohm.

If you would like to know more about the Thomson Bridge – read on:

Resistors above about 1 ohm in value can be measured using a variety of techniques, such as an ohmmeter or by using a Wheatstone Bridge. In such resistors, the resistance of the connecting wires or terminals is negligible compared to the resistance value. For resistors of less than an ohm, the resistance of the connecting wires or terminals becomes significant, and conventional measurement techniques will include them in the result.

To overcome the problems of these undesirable resistances (known as 'parasitic resistance'), very low value resistors and particularly precision resistors and high current ammeter shunts are constructed as four terminal resistors. These resistances have a pair of current terminals and a pair of potential or voltage terminals. In use, a current is passed between the current terminals, but the volt drop across the resistor is measured at the potential terminals. The volt drop measured will be entirely due to the resistor itself as the parasitic resistance of the leads carrying the current to and from the resistor are not included in the potential circuit. To measure such resistances, requires a bridge circuit designed to work with four terminal resistances. That bridge is the Thomson or Kelvin Bridge.