Pictures show that in World War I and
World War II most stretchers were
poles with canvas strung between

In one 1969 photo, ambulance
stretchers were still two poles with a
pad between them. Stretchers have
undergone many improvements in the
last several years.

Now that they are made out of
aluminum ambulance stretchers are
lighter than they used to be. Pads have
become firmer so that CPR is easier to
perform than before. To make it easier
to move over rough terrain, stretchers
now have heavier-duty tires and
wheels. Also, modern stretchers are
curved so that the patient will stay in the

Discovery of Superconductivity
Superconductivity was discovered by a Dutch
scientist, named Heike Kamerlingh Onnes, in
1911 while he was working with the metal
mercury. When Onnes cooled the mercury
down to the temperature of liquid helium (4.2
degrees Kelvin or -268.9 degrees Celsius) he
found that mercury conducted electricity with
zero resistance. The temperature needed to
make a metal superconducting is called the
critical temperature (Tc), in this case 4.2
degrees Kelvin (K). In one experiment Onnes
applied electricity to a copper wire that was
cooled to Tc; one year later the current was still
flowing with no loss of power.

Between 1911 and 1986 new alloys were made
that had Tc's of over 50 degrees Kelvin. The
goal was to reach at least 77 degrees K
because then liquid nitrogen (which is much
cheaper and easier to use than liquid helium)
could be used as a cooling agent. Finally, in
1987 Dr. Paul Chu in cooperation with Dr.
Maw-Kuen Wu reported finding
superconductivity at temperatures over 90
degrees K.
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