Elenco Snaptricity&reg - Manual (Page 41)

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Ohm’s Law - Cold Lamp Project #31
5V
Part A: Build the circuit at left and set the meter (M5) on the 5V
setting. Push the press switch (S2) to measure the voltage across
the top lamp, which is bright.
Part B: Move the meter so it is across base grid locations C2-E2.
Push the press switch to measure the voltage across the lower
lamps, which are very dim. Watch the lower lamps closely as you
press the switch. Initially they are dark, but slowly become dimly lit.
Note: The voltage in Part B will be much smaller; in some cases
it may even be too small to measure with your M5 meter. M5 is
a simple meter, don’t expect it to be as accurate as normal
electronic test instruments.
Operation
The top lamp turns on faster and brighter because it has a higher
current through it, which heats up the filament quickly. The other
lamps turn on slowly and dimly because they have half the
current, which is barely enough to heat up their filaments.
Description
The preceding circuit allowed you to calculate the resistance of
the lamp (L4) by measuring the voltage and current. You did this
while the lamp was bright, but the lamp has much lower
resistance when it is dim.
Assembly
5V
Educational Corner:
The current through the top lamp is split
between the two lower lamps, so you would
probably expect the lower lamps to be half as
bright and to have half as much voltage across
them. Instead, they are much dimmer and
have a lot less voltage.
This happens because a dim light bulb has
less resistance than a bright one! Your L4 light
bulbs have resistance of less than 5 ohms
when dim, and about 15 ohms when bright.
Voltage
Current
Resistance =
Electric Paths Snappy says: when a large current flows through
the lamp, the filament (a special high-resistance
wire) heats up and glows. All wires have higher
resistance when they are very hot.