# Make: the simplest electric car toy, a homopolar motor

**doctorow**#1

**kupfernigk**#3

One thing Iâve noticed in the last 40 years or so is the growing tendency to assign physicistâs names to things (even when not strictly correct). Itâs actually a bit depressing because it reinforces the celebrity view of science, which actually tends to be the work of much larger groups of people. The forces on a charged particle due to the surrounding electric, magnetic or electromagnetic fields derives from Cavendish and Faradayâs original experiments through Maxwellâs equations, J J Thomsonâs experiments with cathode ray tubes, and Heavisideâs correction and modernisation of Thomsonâs formulae. Lorenz really just came up with an equation, and so we should perhaps write âThe Lorentz equationâ, but certainly not the Lorenz effect or the Lorenz force because they were known about and described long before Lorenz. It is a bit like confusing the train timetable with the train.

(This is not specifically about the Lorenz but the nomenclature. For instance, we refer to the âGeiger-Marsden experimentâ but we donât call the deflection of alpha particles the âGeiger-Marsden effectâ or the âRutherford forceâ).

In maths we nowadays tend not to refer to Newtonian calculus (thus omitting Leibniz from the record). Euclidean geometry is a convenient shorthand for âgeometry of three mutually perpendicular dimensions in a flat spacetimeâ, but also because we have no records of anybody elseâs work - Euclid was the one chosen by chance and survivor bias for fame. But where the development of ideas is well attested, itâs rarely the case (Einstein being an exception) that one can point at a name and say âThis is where it all startedâ.

**glenblank**#4

Iâve made both of the simple homopolar motors illustrated in the Homopolar motor article on Wikipedia, but I must say this design is charmingly elegant.

I especially like the asymmetric magnet-wheels so you donât need an infinite roll of foil roadway. (-:

Listening to people who *think* they understand electricity and motors try to explain the dangling-drywall-screw version can be quite entertaining. (And can sometimes reveal just how very little the self-described âexpertâ actually *does* understand.)

**kupfernigk**#6

interesting exercise; calculate the force between two wires by working out the Lorenz contraction of the electrons due to their relative velocity, thus working out the excess or defect charge, and working out the resulting electrostatic force. If you do this, you get the correct answer - with no magnetic fields involved.