The English physicist James Prescott Joule (1818 – 1889) spent a lifetime working to understand the behavior of energy in its many forms. Today the Cabinet of Physics explores the Joule Effect, which concerns the connection between electricity and heat.
Joule showed that the heat energy created by electrical current passing through a circuit is proportional to the the square of the amount of current, and also proportional to “resistance,” a measure of the ease with which current passes through a circuit.
Copper is a pretty good conductor, but nonetheless a copper wire has some resistance, and will heat up if we send many amperes of current through it. In today’s video the copper wire gets hot enough to give off a dull red glow. It’s not a very useful light source, but it’s a hint toward the invention of an incandescent light.
Of all metals, silver is the best at conducting electricity in a circuit like this one,* so if we compare a piece of silver wire to a piece of platinum wire having identical dimensions, the silver wire will have a lower resistance, and the platinum wire will have a higher resistance.
This is made vividly apparent in today’s demonstration. In the chain of linked sliver and platinum wires, the same amount of current passes though each segment, yet the platinum gets hot enough to glow, whereas the silver, having lower resistance, is much less hot, so is invisible in the darkened laboratory.
The Joule Effect is harnessed directly in such devices as toasters, electric heaters, and incandescent bulbs, which intentionally produce heat and light. In devices which put electrical currents to other uses, designers must still deal with the consequences of Joule heating; computers, for example, are intended to handle information, but many computers develop enough heat in their operation that they require fans to provide airflow to keep them from overheating.
The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments. The Foundation’s homepage may be found here, and its Youtube channel, florencefst, here.
*Superconductors are an exception, but they require extremely cold conditions. Also, superconductivity is a Twentieth Century discovery, as yet unknown at the time most of the instruments in the Cabinet were being used to teach physics.
