There are 2 circuits here ; the top one shows two LED's connected in parallel. This means that both LED's get the same voltage of the batteries, this is about 4.5V.
Did you notice that the green LED seems to shine brighter than the red ? This is because our eyes are not so sensitive to red colors.
The serial circuit puts the two LED's in series. They both get only the half of the battery voltage, so they will shine more dimly.
A small question : what do you think of the current flow in both circuits ?
In a parallel circuit, the current will be the sum of both LED's, this is about 15.2 mA
In a series circuit the current would be 7.2 mA, but since we use only 4.5 V, the current will be smaller since both led's already consume 4 V. By measuring across the build-in resistors of 330 Ohms, we find that the current is about 1.07 mA.
If you have a voltage meter, you can also measure these currents.
Later on, we will build our very own galvanometer, which is also capable to measure very small voltages.
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