Hands-On-Physics
Heat & Temperature
Core Project:
TESTING & EVALUATION
Test the Incubator
See whether your incubator works. Rotate the thermometer to a position where
the dial is between the two stationary contacts, not touching either one.
When all the electrical connections are made and the transformer is plugged
in, the bulb should NOT light. Rotate the thermometer slightly so the dial
touches the stationary "heat" contact, and the bulb should light.
The bulb will stay lit until the air in the incubator warms the bimetallic
spiral in the thermometer and moves the dial away from the stationary contact.
Can you feel the box get hotter? When does the heater turn off? What else
do you observe? How can you set the thermostat to a higher temperature.
Improve your Incubator
The thermostat may be rather irregular and untrustworthy. The simple design
makes all the current pass between the dial and the wire contact. This
causes sparking and too much heat which makes the contacts unreliable.
The current can be reduced significantly by using a relay or a transistor
in your circuit. Adding a relay should make your incubator work more smoothly
and reliability. You might reserve this improvement for an extension.
Evaluate your Incubator
Turn off the power and install a thermistor that you calibrated in the "messing
around" section. Install this thermistor so that it measures the temperature
of the air somewhere inside the box not too close to the lamp. Use a pair
of clip leads to attach the thermistor to your meter. Turn on the incubator
and record the thermistor resistance every ten seconds until you think you
have captured enough data. As you take data, note when the light turns on
and off. Graph your data.
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