Paper 6 Physics Help

[geminisign]

Hi there,

I've seen this come up in many papers such as:

Question 1 http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_w11_qp_63.pdf

My teacher never taught me what 'pins' were and I don't really understand what Question 1c means.
Couldn't find any info online, could someone explain the question and experiment to me?

Thanks.