Great question! And here's the answer... 'Direct measurement'
refers to measuring exactly the thing that you're looking to measure,
while 'indirect measurement' means that you're measuring something by
measuring something else. Let me give you some examples that may help
this make more sense.
In physics, there are some things that are very easy to measure
'directly.' These are things like weight, distance, etc. So if I wanted
to measure how long a piece of wood is, I would just measure how long
it is. But let's say I wanted to find out about something that is a
little harder to measure... like how quickly the wind is blowing. I may
not be able to measure the wind's actual speed, but if I had a
windmill, I could measure how much power the windmill is making. Then,
using this information, I could work backwards to figure out how fast
the wind must be. This would be an example where I have to measure
Direct and indirect measurements are also very important in
chemistry and biology. Take, for example, bacteria (little organisms
that are so small that you can't see them without a microscope). If I
wanted to figure out how many bacteria are in a tube, I could measure
it directly or indirectly. To do it directly, I would have to spread
the bacteria out on a microscope slide and count them one by one. This
is /very/ time-consuming. Instead, I could use something called a
spectrophotometer to measure it indirectly. A spectrophotometer works
by shining light in one side of a tube and measuring how much actually
gets through to the other side. Since more light getting through means
there's less bacteria in the tube, I can work backwards to figure out
how many bacteria are there. Another way of measuring it indirectly
would be to measure how much food the bacteria eat in a certain amount
of time. The more they eat, the more bacteria there must be.
There are /lots/ of other examples of how to use direct and indirect measurements... see if you can think of some others!
(republished on 07/18/06)