Re: How fast does the effect of a magnetic field travel

The appearance and effects from the magnetic field are virtually 
instantaneous (technically the field would expand at the speed of light - 
300000000 meters per second). The best analogy is to imagine how soon after 
swiching on a light bulb would all the corners of a room be illuminated. 
Magnetic fields are of the same family as light an so have many similar 
properties. That's why people talk about electro-magnetic radiation.

In practice you could measure the field increasing several meters away and 
the field would reach its maximum maybe a fraction of a second after the 
power to the magnet was "turned on" (only an electro-magnet can be "turned 
on" or "off"). The "slow" (part of a second) buildup of the field would be 
due to the "slow" buildup of power in the magnet - not due to the magnetic 
field expanding from the magnet at a "slow" speed.

Imagine the light bulb again. If you close up film the "turning on" of a 
light bulb and then watch it in super slow motion you would see the 
filament in the bulb heat up from cold and grey to dull red to bright red 
to white to a burning white furnace over a period of maybe 1/10 of a 
second. This is very slow compared to the speed at which the light moves 
away from the filament/bulb.

Hope this helps
Regards
Anthony Barber