Re: electric charge

[Ludwik Kowalski <kowalskiL@MAIL.MONTCLAIR.EDU>]

Herbert H Gottlieb asked:

> What was the date and circumstance of the initial
> modelling of the property called charge?

Is it not true that Franklin argued (in 1760's ?) that Dufey's
"two fluids" theory of electricity should be replaced by the
one-fluid theory? Franklin  is credited for renaming two fluids.
The old adjective vitreous  (glass-like) was replaced by
positive while the old adjective resinous (rubber-like) was
replaced by negative.

The two-fluids theory is closer to our current ideas than
Franklin's theory. According to Franklin an object is neutral
when it has a certain amount of net electric fluid. An object
becomes positively charged when it has an excess amount
of that fluid (with respect to so-called "normal" amount).
Likewise it becomes negative when it has a less than normal
amount of the same fluid.

My guess is that the name "capacitor" was introduced under
the influence of a gas model of electric fluid. Consider two
empty cylinders hidden inside a wall, except for their endings.
Each cylinder has a gauge to measure P. Which cylinder has
a higher capacity for a gas? To answer this question we can
inject equal amounts (n) of gas and measure the pressure.
The ratio of P/n will be larger for the larger cylinder. By using
the ideal gas law we have P*V=n*R*T. P/n can thus be called
the gas-holding capacity. In an electrical analogy V --> P,
Q --> n and the V/Q ratio is called an electric capacity.
Ludwik Kowalski