Re: Alternating Current (postscript)

[brian whatcott <inet@INTELLISYS.NET>]

At 13:46 1/31/99 +0100, Ludwik wrote:
> Is a tiny A.C. component (at high f) is due to light or to the
> antenna effect? A very simple experiment can answer this question.

I suspect it was Ludwik alone who harbored this suspicion concerning
the provenance of the 120 Hz component of the light signal despite
all the other elaborations on the I^2.R theme.

There is a low-cost means of testing his theory.
 Using a phototransistor (RS 276-145A costing $1)
a nine volt battery and a battery clip ( RS 270-324)
and a collector load resistor ( a 4.7kohms value would fit the bill)
an oscilloscope may be arranged to display a signal of 8 volts
on which a sinusoidal variation of 20 mV peak to peak is visible.
(This uses a 40 watt lamp at around 50 cm.)

A black card placed so as to opaque the light path quenches the collector
current and the AC component of the light quite effectively.
 Flashing a strobe confirms the 5 microsecond rise time and fall time
 of the photo sensor suggesting a photosensor bandwidth of about 100 kHz
 in this configuration.
A dimmable halogen lamp shows a pleasing progression from a sawtooth
to a sine wave with increasing brightness.

Brian
brian whatcott <inet@intellisys.net>
Altus OK