Re: Filtri SC

From: andreafr68 <andreafr68_at_despammed.com>
Date: Sun, 15 Feb 2004 11:28:41 +0100

> Ho letto che il concetto di resistenza equivalente ad una capacit�
> commuttata era gi� noto a Maxwell che lo introdusse nel suo trattato
> sull'elettricit� e il magnetismo del 1874.

Confermo. Lo appresi (con un certo stupore)
dalla viva voce di Gabor Temes. Del resto il termine
dovuto alla corrente di spostamento nella formula di
Ampere e' dovuto proprio a Maxwell, quindi egli aveva
gia' intuito che una variazione di campo elettrico e' del tutto
equivalente ad una corrente. Ma non solo: egli aveva anche
capito che un condensatore che commuta equivale ad
una resistenza elettrica. Quest'uomo era veramente un genio!

Ti quoto dal trattato in questione, pag 412
(ma non so l'edizione perche' e' una fotocopia):

" Intermittent Current.

775] If the wire of a battery circuit be broken at any point,
and the broken ends connected with the elctrodes of
a condenser, the current will flow into the condenser with a
strength which diminishes as the difference of the potential
of the plates of the condenser increases, so that when the
condenser has received the full charge corresponding to the
electromotive force acting on the wire the current cease entirely.

If the electrodes of the condenser are now disconnected from
the ends of the wire, and the again connected with them in the
revers order, the condenser will discharge itself through the
wire, and will then become recharged in the opposite way, so
that a transient current will flow through the wire, the total quantity
of which is equal to two charges of the condenser.

By means of a piece of mechanism (commonly called a commutator
or wippe) the operation of reversing the connexions of the condenser
can be repeated at regular intervals pf time, each interval being equal
to T. If this interval is sufficiently long to allow the complete discharge
of the condenser, the quantity of electricity transmitted by the
wire in each interval will be 2EC, where E is the electromotive
force and C is the capacity of the condenser.

If the magnet of a galvanometer included in this circuit is loaded,
so as to swing so slowly that a great many discharges of the
condenser occur in the time of one free vibration of the magnet,
the succession of discharges will act on the magnet like a steady
current whose strength is 2EC/T.

If the condenser is now removed, and a resistance coil substituted
for it , and adjusted till the steady current through the galvanometer
produces the same deflexion as the succession of discharges, and if R
is the resistance of the whole circuit when this is the case,

    E/R = 2EC/T

or

    R=T/2C

We may thus compare the condenser with its commutator
in motion to a wire of a certain electrical resistance, and we
may make use of the different methods of measuring resistance
described in Arts. 345 to 357 in order to determine this resistance."

(Nel paragrafo successivo egli applicher� questa tecnica ad un
ponte di Wheatstone, per misurare effettivamente questa resistenza).

Una brillante intuizione di un genio, che ha trovato delle applicazioni
pratiche industriali circa un secolo dopo.

&
Received on Sun Feb 15 2004 - 11:28:41 CET