Re: How can I construct an EMF to measure its effect on plant growth?

You have a very interesting science project.

First you must know that electromagnetic field always propagates itself
and thus it is also called EM wave. These EM waves are separated in
different domains according to their frequency as follows (roughly):
 - radio waves:           ... to 3E+12Hz
 - infrared waves:    3E+12Hz to 3E+14Hz
 - visible light:     3E+14Hz to 3E+15Hz 
 - ultraviolet light: 3E+15Hz to 3E+16Hz
 - X-rays:            3E+16Hz to 3E+19Hz
 - gamma-rays:        3E+19Hz to ...

One reason why EM waves are separated into these categories is that
there are different methods needed to generate / detect EM waves
from different categories. For example you use an antenna to emit
radio waves and a light bulb to emit light. Visible light can be
detected by the human eye but radio waves not. Instead radio waves
can be picked up by an antenna but not visible light.

It is interesting that all these things represent just ONE phenomenon
and there is only ONE parameter which separates visible light from
radio waves: frequency.

By asking how to produce and measure electromagnetic field you probably
meant radio waves. The answer is: by using antennas, just like an ordinary
radio or mobile phone does. An antenna is a conductor through which
electric charges (electrons) move accelerated. This is done by applying
a variable voltage. Actualy the voltage varies with the same frequency
as the emited wave. Also the length of the antenna is correlated to
this frequency: for higher frequencies you need smaller antennas.

I have never constructed a radio emitter myself so I really don't know
how hard / easy it is. But I would recommend you to search for one
that is already built. You should be able to modify the frequency of the
emitted wave and this wave should have high power if you want to
observe something. Otherwise I suspect nothing interesting will happen.
Also you should make sure that your emitter does not disturb other
"EMF users" nearby.

The problem is that it is too much trouble to do this and probably will
observe nothing interesting. The effect of radio waves on living things
is yet uncertain. Probaly you've heard concern about mobile phones causing
cancer but statistical date to infirm this. Also there were concerns 
about possible negative influence on human body of the electric field
nearby electrical energy distribution network's wires; again statistical
data seems to suggest that this low frequancy fields (50Hz) have no effect
on human body.

It would be interesting however to concentrate on different regions of the
electromagnetic spectrum. For example very high frequency radio waves, 
called microwaves, have a very easy to observe effect on plants and
anything containing water in general. You can "cook" a plant by placing it 
in a microwave oven, which is an emitter of electromagnetic waves in this 
region of the spectrum (above GHz). The plant is heated because it 
contains water and water is resonant to microwave frequencies. This means 
that it is able to absorb the energy of the EM waves.

Another type of EM waves that have a definite effect on plants is visible
light. If you keep a plant without light for a long time it will die. This
is because plants need light for photosyhthesis which is a process by which
they obtain "food" from the minerals extracted from earth. You could try to
see what happens if you keep different plants into different kinds of 
light: red, green, blue, etc.

I hope this helps. If you think something is not clear please don't 
hesitate to email me at: radugrigore@ieee.org.

Here are some other MadSci answers related to your question:
 http:
//www.madsci.org/posts/archives/feb99/917019028.Cb.r.html
 http:
//www.madsci.org/posts/archives/aug98/904184319.Ph.r.html
 http:
//www.madsci.org/posts/archives/oct98/909842532.Ph.r.html
 htt
p://www.madsci.org/posts/archives/dec2000/976039570.Eg.r.html