New welder revisited.

I shall now expose my higgorance once more.

Why is it that a tiny spark across a gap inside an engine, shielded by bonnet and stuff can - if not properly damped - cause an etheric signature that crackles on your radio and TV but a bloody great welding arc that is connected to the mains ne'er causes a quiver?

Regards,

Kim Siddorn

In message , Kim Siddorn writes

It's the frequency, guv, honest.

The welding arc is 50Hz (that is, if it isn't a DC set and it's in the UK). That'd give you a wavelength of 6 million metres - a bit below the ability of the average telly to pick up. The circumference of the earth is only 21.6 million metres.

The spark gap inside the engine is more likely to be in the 20KHz region (see

and that would give a wavelength of 15 thousand metres - still way below telly wavelengths, but some of the harmonics might be getting there. I could be a couple of orders of magnitude out there, and if so, 2MHz equates to 150 metres.

ISTR that a spark gap transmitter (ignition circuit to you) radiates fairly indiscriminately at high energy levels on a broad spectrum of frequencies, unless extreme measures are taken to shield it. I suspect that a 'bonnet and stuff' doesn't really cut the mustard.

is v. interesting and says that telly is about 470 MHz or 64cm wavelength... This is about 2 feet. A half-lambda transmitter aerial would be about a foot long. This, coincidentally, is about the length of an average distributor to spark plug lead in a four-cylinder car engine.

Hope this helps :-)

Pete

In message , Peter Scales writes

This is of course arrant rot. The circumference of the earth is only approximately 40 million metres, as any fule kno. I must of bin thinkin about knots...

The Comission of the French Academy of Sciences originally defined the metre as the ten millionth part of the distance from the pole to the equator, in about 1790. Since then, its definition has become rather more, erm, _defined_...

TTFN

Pete

"Peter Scales" wrote (snip):-

Which, as any fule kno, is a menagerie lion running round the earth. Now will someone put me out of my misery and remind me who that ink stained 'fule' was?

Molesworth.

One day I shall re-write "Molesworth's Pocket Book of Engineering Facts and Formulae" (an excellent little volume), in the style thereof.