When Electricians Were Opticians

Up until the late 19th century there were very few practical applications of experimental findings concerning electricity.  Today, an electric circuit consists of a source of voltage or current, some connecting wires and a load.  The purpose of the wires or cables is to convey the electricity to the load.  The load can be something as simple as an ordinary light bulb.  In the times of the early experimenters there were no loads as such, many of the things which might constitute a load not then having been invented.  A circuit in the experimenters lab consisted of a voltage source, some wire and a galvanometer, with perhaps a cell containing fluid for electrolysis experiments.

The galvanometer as then known was an instrument for detecting and measuring current.  It depends for its operation on simple facts - known from the experimental researches of Lorenz, Laplace, Ampere and others.  Simply put: an electric current in a wire creates a magnetic field which reacts on the wire, and relative motion between a wire and a magnetic field can create a flow of current.   In a galvanometer, a coil of wire connected as part of an experimental circuit creates a magnetic field due to the current and deflects a magnetic needle. 

A question arises quite naturally as to whether the galvanometer, being part of the circuit, is 'really' measuring the current through the circuit or the voltage applied to the circuit.  After all, according to Ohm's law any change in applied voltage will cause a proportional change in current, all other factors being held constant.

It is easy to prove that in a circuit of the type familiar in the 19th century, it is the current and only the current that affects the magnetic needle. Consider the case of a wire carrying a current which deflects a needle.  If a second wire of identical composition and length and connected to an identical battery is placed next to the first wire it is entirely to be expected that the amount of force causing the deflection will double.  In fact, the force is multiplied in a straightforward way by the number of such wires.  Fortunately, it is not necessary to set up lots of identical experimental circuits.  If a single wire carrying current C is formed into a coil of T turns, then the current flowing through any sector of that coil will be C multiplied by T.  That fact was known to the manufacturers and users of galvanometers.  Today we have an expression for the simple relationship between current and the number of turns in a coil: ampere-turns.

Now, it is all very well to be able to detect a current, but how do you draw up and calibrate a scale?  Fortunately, Ohm's law comes to the rescue.

In order to quantify some newly discovered physical force or process, all you have to do - if you can - is to show that a quantity can be derived from the ordinary standard units of mass, length and time.  From experiments of Lorenz, Laplace, Ampere and others

Goldilocks Goes To Monte Carlo
- by way of Hilo.

This article is really about electrical theory