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PASCO OS-8501 Interferometer User Manual

Page 13

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scientific

9

012-02675B

Interferometer

method you find to be easier for controlling the air flow.)

Record P

i

, the initial reading on the vacuum pump gauge.

P

i

= ________________ cm Hg

NOTE: P

i

and P

f

(see below) must be absolute pressure readings. Most vacuum gauges

measure vacuum pressure with respect to atmospheric pressure (e.g., when the gauge reads
34 cm Hg, it means that the pressure is actually 34 cm Hg BELOW atmospheric pressure).
In this case, the absolute pressure reading must be calculated as:

P

absolute

= 76 cm Hg - P

gauge

Of course, your room pressure may vary slightly from 76 cm Hg. If you have a more
accurate way of measuring room pressure, you can use that value to replace 76 cm Hg in
the above equation.

Mark a reference point between a pair of fringes on your viewing screen. Slowly pump out
the air in the vacuum chamber to some convenient pressure level. As you do this, count

m, the number of fringes that pass your reference point. Record

m, below. Also record

P

f

, the final reading on the vacuum gauge.

m = ______________

LASER

VIEWING SCREEN

VACUUM CHAMBER

(front view)

INTERFEROMETER

BASE

BANANA PLUG

VACUUM RELEASE

TOGGLE SWITCH

VACUUM PUMP

MODEL OS-8502

AIR HOSE

VACUUM CHAMBER

Figure 9 EXPERIMENTAL SETUP

P

f

= _______________ cm Hg.