MadSci Network: Physics

Re: induced current?

Area: Physics
Posted By: Ronald Fisch, Physics, Washington University
Date: Mon Apr 15 11:00:33 1996

When the high voltage is first applied, the electrodes act as a capacitor,

which begins to charge up.  Most of the charge should be near the tips of

the electrodes, as long as the air gap is small compared to the box size.

When the electric field strength between the electrodes gets large enough,

the air in the gap starts to ionize, and a current begins to flow.  This

causes the capacitor (i.e. the electrodes) to discharge, and the current

between the electrodes rapidly builds up.  During this process, the

electrodes are acting like a dipole antenna, and radiating electro-

magnetic waves.  Since the time of this process is about a microsecond,

the electromagnetic waves have frequencies around a megahertz.  These waves

are then absorbed by the steel box.  The purpose of the box is to keep the

electromagnetic energy confined; otherwise you would be creating a lot of

radio static!  A steel box is used because steel has a relatively high

electrical resistance.  Since power absorbed goes like RI^2, having a

high resistance keeps the currents in the box lower than if a copper box

was used.

The amount of electromagnetic energy which is radiated depends on the

size and shape of the electrodes (which gives the capacitance), and the

temperature and humidity of the air in the box (which gives the breakdown

electric field).  If you are running 15 amps through the arc lamp, I expect

it will get pretty hot!  I don't think that the initial transient is a

special problem.  The lamp will continue to generate electromagnetic waves

during its normal operation, due to fluctuations in the current. 

How do you get the light out of the box?  A heavy mesh screen will let most

of the light out, while keeping most of the RF energy confined. 

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