The first experiment:

Two coils are connected in series, and then a sliding rheostat, an ammeter, a switch and a power supply (battery pack) are connected in series.

Close the switch, adjust the sliding rheostat, observe the ammeter, make the current reach a certain value, and record the value.

Then use an electromagnet to suck the pin until it can't be sucked. Turn off the switch. Calculate the number of needles attracted and record the value.

The second experiment:

Only one coil is connected in series with the original two coils. Turn off the switch and adjust the sliding rheostat to make the current value reach the value recorded at the beginning. Then use an electromagnet to suck the pin until it can't be sucked. Turn off the switch. Count the number of attracted needles and record the value.

Compare the two values and draw a conclusion:

When the current intensity flowing through the coil is constant, the more turns of the electromagnet coil, the stronger the magnetism of the electromagnet.

The basic thinking method used in this experiment: control variable method

Explore the details:

Why try to connect two coils in series in the circuit first, instead of connecting one coil in series in the circuit first?

Answer: Because the coil has resistance, the resistance of two coils is larger than that of one coil, so the current in the circuit is smaller. This smaller current value can be achieved by adjusting the sliding rheostat in the second experiment. However, if a coil is used in the series circuit at first, and the current limit of the sliding rheostat is not properly adjusted (for example, the resistance of the sliding rheostat actually connected to the circuit is zero), even if the sliding rheostat is short-circuited when two coils are connected in the circuit, the current cannot reach the current value when only one coil is connected in series in the circuit, so the variable cannot be controlled and the experiment fails. In order to avoid failure, the experiment of two coils in series circuit is done first, instead of one coil in series circuit.